Today I had to install the must-have flash switcher extension for firefox (by Alessandro Crugnola) and I found that it’s not compatible with firefox versions after 3.0 …
I trust in sephiroth’s extension going to work properly even in unsupported firefox versions, and however it’s worth the attempt, so, how to install it anyway?
First of all, download the flash switcher extension from the mozilla addons site ( https://addons.mozilla.org/en-US/firefox/addon/5044/ ) and save it on your hard drive.
Now that you have the .xpi file rename it to .zip and decompress it, you’re getting a plugin and content folders, chrome.manifest and install.rdf files.
Open install.rdf with a text editor and examine this tag
here min and max versions are defined for the extension, so just change em:maxVersion from 3.0pre to 3.8 (or whatever) and save the file.
Now select all files and folders (not the parent folder!!!) (plugin, content, chrome.manifest, install.rdf) and zip all together, then change the extension from .zip to .xpi.
Open firefox, File>Open, browse to the .xpi file, install… done and working!! w00t! 
My unsupported (but properly working) version of firefox is 3.6.8, the flash switcher extension version is 2.0.2.
enjoy