Run firefox from the terminal and copy what it says when it crashes here.

Do you have flash installed on your system and how did you install it? (Had firefox download it or did you used the package).
Do you have VLC, gecko-mplayer or any other media player installed?
Did you install these before or after you installed Firefox?
Which version of SliTaz are you using?
Which plug-ins or extensions are you currently using with Firefox?

Any relevant information you supply can make it easier it's to figure out what's causing the problem. Even small changes in about:config can cause crashes hey.

i am running slitaz v 3.2 installed flash through the pkg (terminal) . Don't have any other media player. only have flash plugin installed on FF. and have no idea how to run it from the terminal

Open a terminal window then type the command firefox to start it.

It may throw a couple of gtk+ errors in the terminal (which is to be expected) and the firefox window should pop up normally. Now go to a website where it previously crashed firefox and see if you can make it happen again. It should print out a reason why it crashed in your terminal window when this happens.

I will admit I've had similar issue with firefox about this too. Largely involving pages that heavily used flash (for example having 6 different youtube videos embedded into the same page). It was a once in a blue moon bug for me though and if yours is more common then there is probably some setting, extension or plugin causing it.