Here are some configuration options for Firefox that you can find useful. I have learn about them from Tekzilla.

Editor

editor.singleLine.pasteNewlines (type: integer, {0..4})

0 - pastes CR with no changes
1 - pastes only the first line (default)
2 - replaces CR with spaces
3 - removes CR
4 - replaces CR with ',' (very useful for copy and paste addresses)

Spellcheck

layout.spellcheckDefault (type: integer, {0..2})

0 - no spellcheck.
1 - spellcheck on text input field with 2 or more lines (default)
2 - spellcheck all text input fields.

Tabs

If you have the habit to open many tabs you might want to change the minimal size of the tabs before Firefox start to hide some of them and you are forced to scroll the tabs all the time.

browser.tabs.tabMinWith (type: integer, default 100)

By default the Firefox keeps list of 10 recently closed tabs. If you want to re-open a tab go to "History>Recently Closed Tab". You can change the number of saved undo history by changing the "browser.sessionstore.max_tabs_undo"

browser.sessionstore.max_tabs_undo (type: integer, default 10)

Also there is a keyboard short cut to re-open the colsed tabs "Ctrl+Shift+T".