It is possible that the addon is still compatible, and it's just marked as incompatible because the developer hasn't updated it yet. If this is the case, you can disable compatibility checking by going to
about:config and changing the preference
If it actually is incompatible (i.e. if you disable compatibility checking and it still doesn't work), then you'll need to wait for the developer to update their code, or open up the extension's XPI file (it's just a zip file with a
.xpi name) and modify it yourself. Once you understand the basics of Firefox addon development, look at https://developer.mozilla.org/en/Extensions/Updating_extensions_for_Firefox_4 to get an idea of what changes you'll need to make.