I note you say the error message is "TypeError:Components.classes[cid]is undefined"

This error is often reported and you will need to track down the fault, often with some additional software, one of your Add-ons that is not working properly.

This post by cor-el explains what to do. <-- clickable link --

Note it will be easiest to disable half your add-ons and then try, if the problem goes away or not you then know which half it is in, next disable half of the problematic add-ons and you know which 1/4 the problem is in. That method will be often be quicker than disabling or enabling one add-on at a time.

It will be useful to know what theme or add-on causes your problem, as no doubt others will have the same problem.

Thanks for your help! I suspected it could be an Add-on but wanted other ideas. I disabled all the add-ons which are not compatible with FF 5.0.1 and the problem went away. I enabled them one by one until I found the culprit. The culprit is "HP Smart Print 1.0" Thanks again.

Glad you fixed the problem, and thanks for posting back sating what caused your problem.

Personally I usually find HP are pretty good at keeping software/drivers up to date, although at the moment I have a HP printer that seems to be trying to get updates from a site that my not be secure.