I would like to hear from you why you wanted to stay on an archaic no longer supported OS like Windows 98 SE (only third party non-official updates available), Windows ME and use fx 2?
Software has been developed further, so that is also no longer supported. On the one hand because you have this OS there is lesser specific malware for these OS, unless you protect it properly.
Windows urge you on to grow, functionality and CPU demands urge you to switch.
If you are on Open Software you do not have to grow market-conform. There you have less of these problems. For instance Firefox 2.0.0.0. desktop on Linux use this: http://pyrodesktop.org/releases/compzilla-0.1.xpi (using a windows manager to do this by the name of Pyro).
Because of these developments for Firefox 3 and Firefox Shiretoko we need at least XP to run it, and you know you have to upgrade Win 98 SE to use USB there. Go with the flow!
I know the point has been skirted around in the postings here by bob3160; David was, as usual correct, but in this case not explicit.
You cannot install FF3 on a Win9x based operating system.
I do not believe that those operating systems cannot support it - it is more that the Mozilla developers do not care to undertake any longer the effort needed to make their product work with the older operating systems. They have decided, I suspect correctly, that the declining number of users of those systems and the lack of support for those systems from Microsoft point to an overwhelmingly logical direction.
The avast team are to be commended for their continued support for the Win9x operating systems … I suspect that economics will eventually bring them to the same conclusion as that reached by the Mozilla developers.
Now when you have Nightly Tester Tool you can enforce an add-on like yours to work on Fx3, everything else works just smoothly and also new updates will be found.
With Nightly Tester Tools I run older valuable add-ons on Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1b3pre) Gecko/20081205 Shiretoko/3.1b3pre ID:20081205103154
Shiretoko is the latest nightly trunk build of Fx to be tested and all extensions and add-ons are not upgraded to run there, Nightly Tester Tools enables that. Or very rarely it is that older add-ons are really incompatible and make the browser function erratically or crash. But with your add-on you should be fine, try it and enjoy,