Hello nduin, the latest firefox 48.0 is the last that works for your OS X 10.8.5, 49.0 firefox (released in September) is the one that does not work.

if you have a copy of a previous firefox version try the next : download a new copy and install that firefox copy:

1. Download a copy of the latest firefox from http://www.mozilla.org/en-US/firefox/all.html (full dmg package)

2. Trash the current Firefox application to do a clean install.

3. Install the version that you have downloaded.

Do not select to remove your personal data, your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks or other personal data.

thank you

Firefox 49.0 requires OSX 10.9 or newer.

The current Firefox 48.0 Released on Aug 2nd still works on OSX 10.8.5 www.mozilla.org/firefox/all/ however once Firefox 49.0 is released the Firefox 48.0 is EOL so in order to still get security updates for Firefox you can use the Firefox 45 ESR branch.

ESR channel builds generally get eight updates as 45.8.0 esr will be out when Firefox 53.0 is released. The 45.8.0 esr will then be the last unless Mozilla extends support to help Mac OSX 10.6, 10.7, 10.8 users for a while longer.

See https://support.mozilla.org/en-US/kb/firefox-osx on your options.

Since you are Testing the Beta channel builds you are seeing this change sooner.

Note that you can probably update to Mac 10.11 for free, which you should do anyway as Mac 10.8 is no longer supported by apple and is insecure.