Update: 30.12.2009 Twitterfox is now know as Echofon. It happened long ago though ;).
And I recommend you to update to Twitterfox 1.8.1 coz it’s really improved.
First go here:
<path to your Firefox profile directory>/email@example.com/chrome
[you’ll learn howto find your Firefox profile directory (AKA folder) here. There are too many operating systems. I can’t cover them all :P ]
There is a file called TwitterFox.jar Echofon.jar
Open it & go in to directory called content. There you’ll find the file called twitterfox.js, you better open it with a text editor.
In the top region of the file you’ll find TINYURL_CREATE_API variable. it is set to use tinyurl api by default. I edited it to use my favorite URL shortnet tr.im by editing it to look like this.
var TINYURL_CREATE_API = "http://tr.im/api/trim_simple?url=";
Update: I’m not sure if tr.im is still up. I switched to u.nu (http://u.nu/unu-api-simple?url=)
You can replace http://tr.im/api/trim_simple?url= with the API of your favorite URL shortener.
You need to restart Firefox for the changes to be effective.
Hope it works ;) . Anything not clear? Just ask in comments.
PS: You may not like to try this but I don’t give a shit :P .