The following options have to be changed or added in about:config. Those tweaks can of course be achieved through extensions, but by setting them directly you can abandon some extensions and hopefully conserve some memory.
In order to open a bookmark-folder with mid-click in tabs without overwriting the current tabs change:
To remove the close buttons on the tabs (they still can be closed by mid-clicking on them):
Enable the spellchecker for inputfields and textareas (default is textareas only):
To override the useragent string:
And the last one really made my day: Open lastfm://-links directly in
BTW: Shouldn’t firef*cough* iceweasel open the composer of your mailclient when clicking on mailto-links?