How can I install Selenium IDE on Firefox 5.0?
Selenium IDE 1.0.11 is incompatible with Firefox 5.0.
However, according to Selenium IDE release notes, Selenium IDE 1.0.12 will be compatible with Firefox 5.0.
While Selenium IDE 1.0.12 is not yet formally released, an early build is available for download. (A direct link to the download is provided in the comment thread.)
The link you find there will initiate the automatic Firefox add-on install (after a redirect).