Warning! Some information on this page is older than 5 years now. I keep it for reference, but it probably doesn't reflect my current knowledge and beliefs.
New version of Firefox contains custom PDF files viewer, which is not as good as the standard Adobe Reader. To restore Adobe Reader as default browser for PDF files:
- Enter "about:config" in address bar. Double-click on "pdfjs.disabled" to set it to "true". This disables the new browser.
- Enter Tools > Options dialog, Application tab, locate "Adobe Acrobat Document" and set it to "Use Adobe Acrobat (in Firefox)". This restores Adobe Reader as as browser embedded inside Firefox.