Try using Firefox's safe mode, that will reduce the possibility of some extensions affecting anything, but do NOT make changes at the first dialogur window you see wth check boxes, just click on continue.

The problem may be with the websites, can you give a couple of examples please with URL from the location bar. I have XPs and Firefox 5 available and may try the sites.

Thanks you, it does work in Safe Mode. How do I determine which add-on is causing the problem?

I do not recall anything specific about other extensions preventing pdf use, so you will have to try them all. But first of all check again, just incase you merely needed to clear the cache and it is working again !

The quickest method is to repeatedly slice into 1/2s until you find the guilty add-on Disable 1/2 of your add-ons and see if it works, if not working you know it is one of those you did not disable, disable 1/2 of them. (So you now have 3/4 disabled )

It is also probably a good idea to at the same time check add-ons and plugins are up-to date.

• Use Firefox button -> Add-ons |check plugins are up to date|
  or alternatively the link https://www.mozilla.com/en-US/plugincheck/