Bev is having a problem replying to list so emailed me off list. Replying without context, on and off list...
I've googled and managed to find out how to downgrade a package using apt-get. Which I will come back to in a moment.
I accidentally installed an update to firefox, tried it, missed my add-ons, so downgraded using synaptic. Once I had downgraded, I had to tweak the toolbars etc, but everything worked again. I didn't lose anything. So it's probably worth trying.
Palemoon looks nice to me, like an old version of Firefox. Many firefox plugins work directly in Palemoon, however not all. If a firefox update has been updated to work on the new firefox, it is probable that it will not work on Palemoon. It's worth giving it a go I guess. I also guess that Palemoon won't have as big a team behind it as Chrome or Firefox, so from a security point of view, you're probably best with a big-name browser at least for stuff like shopping!
This link helped on how to downgrade.
https://askubuntu.com/questions/138284/how-to-downgrade-a-package-via-apt-ge...
Shutdown firefox first.
Use apt-cache showpkg <package-name> to show a list of available versions for a package (firefox produces pages!) Do it for firefox, firefox-locale-en and xul-ext-ubufox. My versions for these are 56.0+build6-0ubuntu1, 56.0+build6-0ubuntu1 and 3.3-0ubuntu2.
Apparently you can then do
sudo apt-get install <package-name>=<package-version-number>
so I guess you'd do all three on one line:
sudo apt-get install firefox=56.0+build6-0ubuntu1 firefox-locale-en=56.0+build6-0ubuntu1 xul-ext-ubufox=3.3-0ubuntu2
(all on one line).
If that works, then see if firefox works & has downgraded. If all's OK, then try
apt-mark hold <package-name> which "holds" the package at the current version, preventing automatic upgrades.
sudo apt-mark hold firefox firefox-locale-en xul-ext-ubufox
Hope this helps.
Steve