Comment 25 for bug 1940417

(In reply to Jeroen from comment #8)
> I'm not exactly sure how to install a Firefox beta in a separate folder and run it. If you can help me with that, I'd be happy to help.

No need for it to be a beta. Just download the upstream build from https://www.mozilla.org/firefox/download/thanks, unpack the compressed archive in a temporary folder, this will create a "firefox" folder in there, and from that folder just run `./firefox`.
Once you've tested and reproduced (or not) the bug, you can safely delete that temporary folder.