Starting with version 1.2 of mozilla, a bug was introduced into the
"fast load" (startup) feature, that can cause mozilla to freeze,
usually when you try some action such as sending email or printing a
document. If your browser freezes, the first thing you should do is
kill the browser and try again (just in case it was a one time
thing). If it still freezes, try removing the fast load file as
follows:

1. Quit (or kill) mozilla.
2.
*On UNIX systems:*

*rm ~/.mozilla/*login*/*XXXXXXXX.slt*/XUL.mfasl*
where *login* is your login name and * XXXXXXXX.slt* is
some random directory name ending in .slt that you will
need to determine by looking in ~/.mozilla/*login*

*On Windows 95/98/ME/NT/2000 systems:*

Delete the file U:\.mozilla.w2k\User\puegzo40.slt\XUL.mfl

3. Start mozilla and try again. If it still does not work then
you should start over with fresh mozilla preferences. How Do I
Start With Fresh Mozilla Preferences?
Click here to find out.