Search Gmail in Firefox
Go to this site and install Gmail Search.
Search Gmail in Internet Explorer
Download the new Google Toolbar 4.
Search Gmail in Opera
Go to Help/About Opera, copy the folder location for "Opera Profile", open it. Edit search.ini, go to one of the entries [Search Engine n] (for example, [Search Engine 2]) and replace the first two lines with:
Name=Gmail
URL=http://mail.google.com/mail/?search=query&view=tl&start=0&init=1&fs=1&q=%s
More about Gmail:
Gmail Chat in Opera
Switch from Yahoo Mail / Hotmail to Gmail
5 fast ways to check your Gmail account
Marissa Mayer, Google VP, prefers Pine to Gmail