I installed Firefox 10 into
~/firefox. I've downloaded Adobe Flash Player into
~/flashplayer. In the past, I could symlink
~/firefox/plugins. But now, when I look, there is no
plugins directory inside
firefox. So where does Firefox look for its plugins?
I had a similar problem when installing firefox 16 on OpenSuSE 11.4 x86_64. I had firefox 5 installed with the distro and decided to just download the latest version from mozilla. The browser worked, but the java plugin could not be found even when placing a symlink to 1.6_37 version of the oracle java plugin in /plugins, /usr/lib/mozilla/plugins, or /.mozilla/plugins.
After reading about problems with the 64 bit java plugin, I decided to update firefox through yast and it found two java plugins: an IceTea and just fine. Yast installed some compatibility packages to make it work. Furthermore, it turned out I had to use the IceTea plugin provided by OpenSUSE as the Oracle plugin crashed firefox.
Firefox still looks in
~/.mozilla/plugins. A likely explanation is that you are running a 32bit version of firefox on a 64bit installation. Mozilla only packages 32bit versions of firefox, so if you download the "official" binary package from Mozilla that's what you get, regardless of whether you are running 64bit GNU/Linux. The resulting situation is that your distro-provided plugins are 64bit, which your 32bit browser cannot use.
If you were to download the firefox source and compile a 64bit version, you would find that it would work just fine with the plugins that you have.