For starters you should upgrade to Reader X if you're on Windows 7.

You don't say what version of Firefox you're using, but as of today, the plugin for Adobe Reader X and Adobe Acrobat X works with Firefox 5.01 or earlier. Due to Mozilla's strange decision to issue a new major version every couple of weeks, it's almost impossible for plugins which ship as part of a larger product to keep up, so the Acrobat X Family won't install on FF6 - this should be rectified soon, but by then Mozilla will probably be on FF7 or 8!

Adobe Acrobat 8 is not supported on Windows 7, so it's not surprising to find issues.

My advice is to remove Adobe Reader 9 completely, and install the free Adobe Reader X software, making it the default application to open PDF files. If you need to use Adobe Acrobat you can continue with version 8 and hope it works, or purchase Adobe Acrobat X (which is fully-certified for Windows 7).

As of today, the Extension for Adobe Acrobat and Adobe Reader will not work with Firefox 6. Every time Mozilla updates their software, vendors must rebuild their codes, as Mozilla's installer for extensions insists on a version-compatibility check; even if the XPI code itself will run, it is blocked if it's not specifically tagged for the browser version in use (because Mozilla's rules for inclusion in the extension catalog require that a tagged version must have been tested against the maxVersion number, we can't simply say "maxVersion = 9999").

The browser plugin still works just fine, so PDF files can be viewed in Firefox 6 using either Adobe Acrobat or Adobe Reader X - what is missing is the toolbar for creating PDF files of the current page. You can achieve the same thing from within Acrobat X Pro via the Create > PDF from Web Page tool.

Adobe releases updates for the Acrobat Family approximately every quarter, and it is totally unrealistic to expect that frequency to increase. The plugin still works, users can view PDF files within Firefox as they could before, but pushing an update to every Adobe Reader and Acrobat user simply to replace the XPI file will cause no end of complaints from enterprise administrators. All the leading vendors are in the same position - including Google, Microsoft and Real Networks - their plugins are working but their extensions are blocked. Users can hack the XPI file to fake the header tag, but the vendors cannot.