Firefox 5 was released before schedule with far less fanfare than Firefox 4. Firefox 5 is now also available for Ubuntu. If you have already upgraded to Firefox 5, you might be able to enjoy all the latest features it has. However, a lot of users are complaining about Firefox 5 crashing after installation among other problems.
Though there is no specific reason for Firefox 5 crashes, you could try out one of the several solutions listed below to see if it helps to solve your problem.
One of the main reasons behind Firefox crashing could be a bad installation. This could be caused due to several reasons. One of the solutions would be to backup your Firefox data and then completely uninstall it from your system.
Once you have done that, download Firefox 5 again and do a clean install. This should more likely than not stop FF5 from crashing.
Disable All Extensions and Themes
When Firefox is updated, some outdated extensions could cause problems which might lead to crashes. Firefox add-on manager provides you an easy way to disable out-dated extensions. Make use of the add-on manager to disable those extensions and restart Firefox to see if that solves your problem.
Use Different Profile
If you are not keen on uninstalling and installing Firefox or cannot disable the Firefox add-ons, you could try creating a fresh new profile in Firefox. You will instructions on creating and managing new Firefox profiles in this Mozilla support article.
Now you might not be a programmer, but you can run some basic crash debugging by going through the points listed in the Mozilla support topic. You can submit your crash reports to Mozilla or contact them to see if there is a solution to your problem.
If none of these work, try downloading an older version of Firefox and installing it to see if it solves those crashes. If not, you could switch to a temporary browser like Google Chrome, Opera or Internet Explorer 9 till an update is available from Firefox.