The following add-ons are invaluable for anyone developing mobile sites.
XHTML Mobile Profile
This add-on allows FireFox to support the XHTML Mobile Profile mime type
This add-on allows FireFox to support WAP 1.0. Just in case you are unfortunate enough to need it.
Just as essential for developers of mobile sites as those viewed on the desktop.
Because validation is even more important on mobile as some browsers really struggle or even throw errors with invalid markup.
An essential tool for all web developers.
Yahoo's tool for identifying ways to improve the performance of a pages load time (Requires FireBug)
Google's tool for identifying ways to improve the performance of a pages load time (Requires FireBug)
User Agent Switcher
Allows testing how a site will be displayed on various devices. (Assumming that pages are adjusted for sites based on the UA.)
Because sometimes you need an easy way to test, and change, headers other than just the User Agent.
Live HTTP Headers
Another way to see what headers are being sent.