I put flash player 10.1 in the plugin directory, but Firefox displays version 10.0
I updated Firefox from 3.5 to 3.6 and was then asked to update the flash player. I downloaded the tar archive and copied libflashplayer.so to /usr/lib/browser-plugins, after backing up the file that was already there. Firefox tells me it's 10.0 R42, which is what I had before the update.
Every time Firefox opened
== I downloaded the new version of Adobe flash player
The usual location of the Flash plugin is /usr/lib/flash-plugin/libflashplayer.so
You can see the installed plugins on the about:plugins page. If you set the pref plugin.expose_full_path to true on the about:config page then you see the full path of plugins on the about:plugins page. It is best not to leave that pref set to true as it exposes that full path to web servers, so reset that pref to false after you are done with the about:plugins page.
You can open about:plugins and about:config via the location bar, like you open a website (about: is a special protocol to access some build-in pages). If you get a warning when opening the about:config page then you can confirm that you want to continue.