You can probably hack the extension to support FF5. There haven't been too many changes that should affect this.
1. Open "Documents and Settings\<user name>\Application Data\Mozilla\Firefox\Profiles\<profile>\extensions
2. Find the folder for the unsupported extension, there should be a file called install.rdf inside.
3. Open the file and change the <em:maxVersion> to 5.*.
Might have missed something, but that did the trick for me.
And hence the stupidity in adopting Chrome-style meaningless major version-numbers when what is released is only a minor update. I know the IT industry is highly fashion-driven, but that doesn't mean following fashions doesn't have drawbacks.
This is Firefox 4.1 at best, but released as Firefox 5 with all the needless addon breaking headaches that will bring, just because Mozilla wanted to be more "fashionate". Great job guys. Great job.
I guess this another thing we can thank the Chrome team for.
Point releases in the past have broken extension compatibility, so asking that it be 4.1 instead of 5 wouldn't necessarily have the kinds of tangible benefits that you seem to think it would.