If the plugin's author didn't update their plugin, then it can only run on specific versions of firefox. You can force this to be bypassed in fire fox. Here's how:
1. type about:config in the URL bar.
2. click on i'll be careful
3. right-click and select new -> boolean
4. now enter extensions.checkCompatibility.version_number using your current version number. Set the value to false.
For example, if you are using Firefox 10.0, you'll need to set the name to extensions.checkCompatibility.10.0 and if you are using Firefox 11.0 beta, you'll need to set the name to extensions.checkCompatibility.11.0b. Similarly if you are using Firefox 12.0 alpha, you'll need to set the name to extensions.checkCompatibility.12.0a and so on.
5. If it still doesn't want to install, then proceed to 6.
6. repeat step 3 & 4, but create the key called extensions.checkUpdateSecurity. Set it to false also.