View the Flagfox version history with Firefox 4.0b2pre. The top couple files correctly list compatibility but still say "Not available for Firefox 4.0b2pre". Similar problem with latest SeaMonkey 2.1a3pre. Opposite problem also exists. View the same page with Firefox 3.6.6 and scroll down to some older versions that don't support it. No incompatibility warning. This reminds me of bug 573906. Maybe it's only checking current public version? Can't test the first part on preview because the appversions list is out of sync, however the lack of warnings for older versions is observable on both.

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:2.0b2pre) Gecko/20100701 Lightning/1.1a1pre SeaMonkey/2.1a3pre - Build ID: 20100701101320 With this SeaMonkey build, I see the spurious "incompatibility warning" on the SeaMonkey "All Flagfox versions" page https://addons.mozilla.org/en-US/seamonkey/addon/5791/versions/ but not on the corresponding Firefox addons page https://addons.mozilla.org/en-US/firefox/addon/5791/versions/ . This suppression might be intentional.

Unless the requirements changed, the all-versions page is not supposed to do any version detection anyway.