Firefox has a setting "Don't load tabs until selected" (see How do I make Firefox 13 Load All My Tabs on Startup or when Resuming Reload). I like that behaviour.

I am searching for a way to "deload"/deactivate a tab manually for a session (until I reload it).

It should stop all running JavaScript functions and plugins (like Flash). The whole webpage content may disappear until I reload/re-activate the tab, but that is not a requirement. The title has to be displayed as tab label (like it is the case with the startup setting, too).

The workaround would be to restart Firefox and don't switch to the tab I want to be deactivated. This is pretty annoying, of course.

EDIT: Here is what I found so far

(thanks, @bytebuster!)

• BarTab
  • no longer being maintained (see why)
• BarTab Lite
  • seems to miss this functionality from BarTab
• Dormancy
  • experimental; comes with warning that it "may eat your session"
• Tab Mix Plus
  • Feature request: Unload Tab feature
• Tab Utilites
  • seems to offer this functionality only for automatic unloading
  • Feature request: Add "unload tab" to tab context menu.
• UnloadTab
  • removed from addons.mozilla.org (who knows why)