I had soooo much trouble with this but did some research and found a solution that sort of worked. THEN today I tried to save some PDF files and again I couldn't. Arggghhh. Took some deep breaths and tried all the options again.

Here's the one that seems most efficient and is working today (I swear it didn't work when I first upgraded to Firefox 4, but if it's working now -- that's a good thing.)

Change the Action setting for Adobe Acrobat Documents in Tools > Options > Applications to Use Adobe Acrobat (in Firefox) . Then you can save your file with the icon at the bottom of the page.

Thanks- that is working for me

Nitro PDF Download 3.x is not compatible with Firefox 4.x. I only discovered this after much frustration trying to open PDFs within Firefox. The only thing that PDF Download would do is save the file outside the browser.

Once I disabled PDF Download in the Add-Ons>Extensions menu, all my functionality returned. However, I had Adobe Reader X installed as well as Acrobat Pro, and even though I had Acrobat Pro listed as the default program to open PDFs, Firefox used Reader instead. If you have both a Acrobat and Reader installed, you should delete Reader. Then Acrobat will function within Firefox as expected.

i tried to do that but it didn't work, the more easy way i found that to go the menu bar click file then save page as .... the PDF file will be saved, i tried this and it work

In my case, the saving problem with PDF files is not reproductible. But once it produces, I hit on Print icon then I cancel the printing. This operation (1- Print, 2- Cancel) activates again Save button. This is reproductible.