I'll note the installation switches, that will come very handy indeed

However, if you make that, you need to set the current user to have rights over the /usr/bin/mozilla-firefox folder, or otherwise your firefox won't be able to update his components on the occasion.

Another remark : no surprise that YOU might not have had problems with the mplayer plugin, that seems only related to the latest abdlock version present on addons.mozilla.org website. Uninstall adbock, or install a latest version gotten from the forum of the adblock dev team, and the problem is gone.