Since Firefox was upgraded to version 43 on my Ubuntu 12.04 LTS machine, it "reliably" crashed on some pages. The easiest example being youtube.com, but many other pages also.
After trying many things which didn't work (disabling all extensions, all plugins, creating a fresh new profile), I decided to downgrade Firefox to the previous version.
But the previous version is hard to find!
The normal repository only contains version 43 for Ubuntu 12.04. The other versions in that folder cannot be installed because they depend on later versions of my libraries...
Finally, Google found me the previous version with this search:
Which led me to
The rest is easy:
Remove firefox (not "purge" as is often recommended, because that may remove your profile with all your bookmarks, extensions, settings, etc.!)
sudo apt-get remove firefox
Get and install the wanted version:
cd /tmp wget "https://launchpad.net/~ubuntu-mozilla-security/+archive/ubuntu/ppa/+build/8220818/+files/firefox_42.0%2Bbuild2-0ubuntu0.12.04.1_amd64.deb" sudo dpkg -i firefox_42.0+build2-0ubuntu0.12.04.1_amd64.deb
Prevent future upgrades (but also prevents security upgrades!)
sudo apt-mark hold firefox
It may be time to look for a better browser than Firefox, but in the meantime, this works