Adding a Search Plugin for Firefox Via Extension is done using bundles, putting your .xml OpeSearch in /searchplugins/ directory and Firefox will automatically add them when the Extension is installed.
See https://developer.mozilla.org/en/Bundles and also my own question Including a Search Plugin in my Firefox Extension
Well, it worked fine, until i started using Firefox 6. It seems that Firefox 6 does not read the /searchplugins/ directory anymore and does not add the Search Plugin. It might be also Firefox 4 and 5, i jumped from 3.6 to 6 directly.
Any idea if this is a bug in Firefox 6? Any idea how to by pass it and install the SearchPlugin Anyway?
I think that the only relevant difference between Firefox 3.6 and Firefox 6 (Firefox 4 actually) is that by default extensions are no longer unpacked on installation, instead they are placed into user's profile as XPI files. This works for most extensions but particularly search plugins won't work from an XPI file. Which is why you have to add the unpack flag to your install.rdf file:
<Description about="urn:mozilla:install-manifest">
<em:id>...</em:id>
<em:unpack>true</em:unpack>
...
</Description>