I use a number of addons in Firefox .. but never used it.
I do however use a couple that are not 'officially' for Firefox 5.
The addon can be modded to allow it to be installed .. You will just need to test it to see if it works ok. The worst that can happen is that you have to delete the intallation folder for this addon before you start Firefox .. which is easy enough to do.
I am using one addon in 5 that was never certified even for Firefox 5!
There is a new addon that is a compatibility 'helper' .. which is not brilliant.
Let me test GMarks for you in 5 for an hour .. I'll let you know what do do to MOD it so you can try it.
Al.