Old Firefox releases (binary and source) are very much available for download, though whether you can run them on modern operating systems somewhat varies. And you almost certainly can't compile some of them with a modern compiler, though a compiler from the era of the release should work...
Anyway, https://ftp.mozilla.org/pub/firefox/releases/ has links to all the Firefox release binaries and corresponding source snapshots starting with version 0.8. It includes a number of the betas as well (so for example has the Firefox 58 beta builds already).
>And you almost certainly can't compile some of them with a modern compiler,
I wonder how long and difficult it must be to get all the required dependencies to compile a modern web browser...
[two hours of obscure error messages later]
"You have v2.3030392.302 of dependency 113, you need to compile v2.3030392.300 in 32-bit first, then build the 64-bit version of v2.3030392.301 then ..."
You get all the compiler errors fixed and then it's on to round two... linker errors.
Maybe old browser versions should be archived as container images with all the necessary shared libraries. Then the only interface that might break them is the kernel interface, and we know Linus' stance on breaking userspace.
I've created dev environment VM's with classic operating systems (XP, etc) because my job involves updating lots of really old code (most clients only update when they have to, every 5-10 years).
I could definitely see someone doing that with Firefox. They may already exist. I've just, from experience, had so much trouble with compiling large codebases.
On the bright side, the larger a codebase, often, the more popular. And the more popular, the more likely people will have EXACT dependencies listed.
For the current version, it's not too bad if you're on a "popular enough" OS. For example, Firefox has a bootstrap.py script for Linux which will install the right packages on either Ubuntu or Fedora Core. See https://developer.mozilla.org/en-US/docs/Mozilla/Developer_g...
Anyway, https://ftp.mozilla.org/pub/firefox/releases/ has links to all the Firefox release binaries and corresponding source snapshots starting with version 0.8. It includes a number of the betas as well (so for example has the Firefox 58 beta builds already).