These arent extensions these are tweaks for how Firefox behaves.
If you type "about
:config" in the address bar (without the quotes) it will bring up a page asking you if you want to continue and TO BE CAREFUL. Altering random values in here can mess your Firefox right up so dont mess around but dont be scared of it, if you change nothing then nothing will happen.
The warning page
This is the geeky looking page where all the values are
Dont be scared as long as you dont change anything nothing will happen.
If you do want to change things then read on.
For example you can set Firefox to open search results in a new tab instead of the current one.
If you type this into the FILTER at the top of the page
browser.search.openintab
It will bring up this. This means when you type into the search box and hit enter, Googles (or whoever you have as your default search) results will open in the current page
The
STATUS=DEFAULT and
VALUE=FALSE 
If you double click anywhere on the text it will change to this
The text will now look bold, the
STATUS will be
USER SET as you have set this yourself and the
VALUE will have changed to
TRUE.
Now if you type in the search box and hit enter the results will open in a new tab and not the current window.
There are loads of these tweaks here
Optimizing Firefox 3 Hacks And Tweaks
If you see ones you think might be useful type the first 2 words like
browser.tabs
into the FILTER and it will narrow down the amount of results.
To change it back from
VALUE=TRUE to
VALUE=FALSE double click the text
DISCLAIMER
Always double check that you are altering the right value as it can mess up Firefox and Im not responsible for any of that, OK?
