Firefox 3.5 override
If you need to install Galaxyplugin 2.1.x into Firefox 3.5 use this addon ( https://addons.mozilla.org/de/firefox/addon/6543 ) to override any compatibility issues Firefox might bring up.
Download and install Nightly Tester Tools
existing installation of galaxypluging: go to Extras--> Addons and click "override compatibility in the bottom right
new installation of 2.1.9 install as usual and click "force install" (screenshot below)
Enjoy Galaxyplugin 2.1.x with Firefox 3.5
you may delete Nightly Tester Tools as soon as you have a compatible version (either 2.1.10 or 2.2.x) at your disposal.