Some people have reported that Firefox 4 sometimes doesn't detect an existing Adobe Reader X (version 10) installation, and they have to reinstall Adobe Reader after installing Firefox 4. I haven't followed those threads carefully to know whether this has been fixed.

Depending on how you like to work with PDFs, you could consider opening them in Adobe Reader rather than in a tab in the browser. To set that, you open Adobe Reader, go to Edit > Preferences, click Internet in the list box, then uncheck "Display PDF in browser".

Tried the reinstallation of Adobe after Firefox 4.0.1 was installed and that didn't make a difference. Still could not view my bank statement. As of now Firefox 3.6.17 is working fine. Statements open as they should.

Try this...this fix was posted in a different thread...

Open Adobe Reader X Click Help --> Repair Adobe Reader Installation

This fix worked for me.

Tried it all and nothing works. Went back to 3.6 and that worked for a while and now it stopped. Not sure what is going on with Firefox and bank statement. I know I am not alone and maybe someone will figure it out down the road. The only thing working for me right now is IE. Opens all my Bank Statements without a problem.

Was also having trouble with pdf's in browser and read most posts. I eventually found that Acrobat 9 pro seemed to be my default! and then found that Adobe reader was not updated. Updated reader to reader X and enabled reading in browser; then went to Acrobat 9 pro and disabled reading in browser, voila, now working fine!!! IE opened them but could not save from IE; Chrome and Safari were the same as Firefox, so would suggest an Adobe mismatch. Cheers.

Tried everything suggested with no luck. Then someone suggested Firefox 5.0 and bingo it worked. Had to set up as if it were a new setup but when finished it worked fine. Latest Firefox 5.0 and newest Adobe Reader and I'm back in business.