If you are the only user then having Plugins in a created plugins folder at /home/username/.mozilla/plugins/ is a option.

Note if Firefox is still running when you place Plugins in either place then either restart Firefox or even type about:plugins in the Location (address) bar to scan for Plugins then go to Addons Manager > Plugins.

Note the Firefox 22.0 is rather old and vulnerable (released back on June 25, 2013) as the current is Firefox 50.0 and 45.5.0 ESR.

As of Firefox 46.0 and later GTK 3.4 or preferably newer is required for Firefox to run and as a result GTK3 themes is needed as GTK2 will not work and Firefox will look unthemed and have bugs like missing scrollbar arrows.