How To Open Source Code In Mozilla Firefox

Opening source code in Mozilla Firefox 1.4 is easy. Simply load your target page, then look for the View drop-down list from the menu bar. Select Page Source from the list of options, which will automatically open a new window and load the HTML source code in that window. This new window is part of a simple interface created by Mozilla Firefox developers for viewing web page source code; you can scroll up and down in this window to look at any portion of the code. Which Tag is currently selected (highlighted) in the image below so you can see the tag line with tag description on the bottom of image.

To view the source code of a web page in Mozilla Firefox, follow the steps below. Click the Menu icon in the upper right-hand corner of your browser window. This icon looks like three stacked lines. Select Web Developer from the drop-down menu that appears. The Web Developer menu gives you various tools to work with the page’s HTML and CSS

Limitations of View Source

There are limitations to what View Source does for you that you need to be aware of.

Error reporter ≠ validator

View Source only reports parsing errors, not HTML validity errors. For example, putting a <div> element as a child of a <ul> element isn’t a parse error, but it is not valid HTML. Therefore, this error will not be flagged in View Source. If you want to check that HTML is valid, you should use an HTML validator, such as the one offered by W3C.

Not all parse errors are reported

Even though all the reported errors are parse errors according to the HTML specification, not all parse errors are reported by View Source. Among the errors that don’t get reported:

  • Bytes that are illegal according to the document’s encoding aren’t marked as errors.
  • Forbidden characters aren’t reported as errors.
  • Errors related to the end-of-file aren’t reported.
  • Tree builder errors relating to text (as opposed to tags, comments, or doctypes) aren’t reported.
  • Parse errors related to xmlns attributes aren’t reported.

Firefox is focused on privacy

The maintainer of Firefox is the Mozilla foundation, a non-profit organization not motivated at all by your personal data. Mozilla doesn’t care about what you search for, what websites you visit, or how much time you spend on the web. That’s just not Mozilla’s business model, but it is expressly the business model of many other popular browsers.

Even if you don’t object to a browser tracking your activities for privacy reasons, you may have had the experience of making that one-time purchase of an unusual gift, only to have every site you visit try to sell you that item for the rest of your life. The Internet is big, so it sometimes seems like a good idea to restructure it based on users’ interests. I admit that if all of my searches could have “open source” appended to them, it would probably provide more relevant results. But then again, I’d rather opt in to that kind of optimization rather than have it, along with many other unknowns, being decided for me, outside my control.


Privacy in Firefox

Privacy in Firefox (Seth Kenlon, CC BY-SA 4.0) 

Mozilla has this policy: Take less. Keep it safe. No secrets.

Never one to stop short, Firefox also offers account monitoring, an optional service from Mozilla that alerts you if any of your online accounts become compromised through a large-scale data breach. Additionally, Mozilla offers a paid VPN service using open source Wireguard software so you can browse safely from anywhere.


bitwarden firefox extension

Undoubtedly, one of the best password managers available out there.

Bitwarden is an open-source password manager offering a variety of features. It focuses on providing competitive open-source solutions.

The password manager add-on available for Mozilla Firefox is no less than any other similar offerings. You get all the essential functionalities starting from generating passwords, managing your vault, along with some advanced options right through the extension.

In my use case, I don’t find the extension lacking anything at all. And, you should try the add-on if you haven’t already.

XML syntax highlighting

View Source uses the HTML tokenizer when highlighting XML source. While the tokenizer has support for processing instructions when highlighting XML source, that’s the only XML-specific capability provided. Because of this, doctypes that have an internal subset are not highlighted correctly, and entity references to custom entities are also not highlighted correctly.

This mishighlighting can be seen by viewing the source of Firefox chrome files (such as XUL documents). However, this shouldn’t be a problem in practice when viewing typical XML files.

Firefox can use containers

It may be hard to believe, but there was a time when leading web browsers did not feature tabs. In the late 90s and early 00s, when you wanted to visit two web pages simultaneously, you had to open two separate browser windows. Firefox (and the Mozilla browser before it) was an early adopter of the tabbed interface.

Tabs are expected in browsers now, but through Firefox’s extensions, there’s been an interesting new twist on the power of the tabbed interface. Developed by Mozilla itself, the Firefox Multi-Account Containers plugin can turn each tab into an isolated “container” within your browser.


Containerized tabs in Firefox

Containerized tabs in Firefox (Seth Kenlon, CC BY-SA 4.0) 

For instance, say your employer uses Google Apps, but you don’t trust Google with your personal information. You can use the Multi-Account Containers plugin to isolate your work activities so that Google touches only your professional life and has no access to any other part of your life.

You can even open the same site with two different accounts, and as a bonus, the tabs are color-coded, so it’s useful whether you’re looking to isolate sites or just add new visual cues to your browser.


vimium firefox

An open-source tool inspired by Vim keyboard shortcuts, originally popular for Chrome, ported to Firefox.

The add-on is a work in progress for Mozilla Firefox, with no recent activity. However, as an experimental add-on, it still has excellent user reviews.

This add-on lets you use keyboard shortcuts to improve your browsing experience. For instance, you can set shortcuts to scroll, view source code, enable insert mode, browse the history, check downloads, and more.

If you are comfortable with keyboard shortcuts, this add-on should be on top of your bucket lists to try if you haven’t.

You can find its GitHub page and explore several customized versions (forks) of it as well.

uBlock Origin

ublockorigin firefox

If you want to get rid of several dynamic elements in a website to improve the browsing experience, uBlock Origin is a fantastic content blocker for the job.

For starters, it blocks a wide range of ads, trackers, pop-ups, to make the web page faster to load. It should come in handy if some web pages stutter when it loads up in your browser.

You can also choose to selectively block/allow JavaScript if a website does not function as it should. It also features filter lists to help you enable aggressive blocking or minimize blocking to balance the web browsing experience without breaking websites.

Firefox user interface design

As much as we humans tend to get excited about new things, there’s just as much comfort to be found in something familiar and reliable. Firefox has updated its interface over the years, and it’s had its fair share of innovations that are now unofficial industry standards, but overall it has remained very much the same. Its user interface retains all the standard conventions you already take for granted.

When you download a file, you’re prompted for how you want Firefox to handle the file. At your option, you can open the file in an appropriate application or save it to your hard drive. You can prompt Firefox to remember your choices for the future or have it continue to prompt you.


Firefox user interface

Firefox user interface (Seth Kenlon, CC BY-SA 4.0)When you need an application menu, you can find it in a modern-style “hamburger” menu, or else you can press the Alt key to show a traditional menu along the top of the Firefox window.

Everything in Firefox is familiar, whether you’re a long-time user of Firefox or not because it builds on years of user interface design. Where it can innovate, it does, but where it’s counter-productive to change something intuitive, it refrains.


languagetool firefox

Note: For this list, we try to recommend Firefox add-ons that are totally open-source. But, this is an exception as a non-foss add-on, where the service is originally open-source, but the extension is not.

LanguageTool is an open-source grammar and spellchecker that respects your privacy, making it a decent alternative to the likes of Grammarly and others. It is free to use, with an optional premium upgrade for advanced correction features.

It should be good enough for basic spellcheck and common grammatical mistakes. As I write this, I have LanguageTool Firefox extension active. Not just a privacy-focused, open-source alternative, it works super quickly without impacting your writing experience.

The server-side is open-source but unfortunately, the add-on is not open-source. They clarified the reason as they do not want competitors to use the add-on and contribute nothing in return (more in their forum post).

However, Mozilla gets access to the source code to review with every release, which makes it a recommended add-on to try.


Did you know that you can open source code just like any other web page in Mozilla Firefox? The view-source pseudo protocol may be used to view the source of any non-file URL.

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