I upgraded to Mountain Lion and now have trouble with Firefox, saying I need to open in 32 bit page. Don't know how to do that. Also won't open PDF files. May change to different browser.
Firefox for Mac is now 64 bit. What do you mean, a 32 bit page? The code on web pages has nothing to do with what CPU you have.
If you cannot get Firefox to handle PDF files on its own, you can have it use a helper application instead. Preview can open PDF files.