The error message suggests that the file (pdf) that you're trying to download cannot be saved in the folder Firefox tries to save it in. This can be due to several things - it could be a non-existing folder (could be a problem with the updated application), the hard drive could be too full to store the file (less likely if you can generally surf the net without error messages) or it could contain an error. To complicate things a little further, it seems to me that you have (at least) 3 different PDF-handling plugins in Firefox from different programs: PDF-XChange Viewer Netscape Gecko Plugin Adobe PDF Plug-In For Firefox and Netscape "9.3.2" Zeon PDF Plugin For Mozilla This shouldn't matter too much, thou, as it will most likely simply use either the latest installed or the first one it finds when looking up associations for files of type 'pdf'. First, in Tools > Options... > tab General > section File download, select "Save files in" and Browse your way to a (non-writeprotected!) folder on a drive where you're certain there is enough free space - like the Desktop folder to make the files end up on your Windows desktop. Second, under Applications as you mention yourself, find all document types related to PDF files, and set these to one of the programs. Check the entire list to be sure there isn't one hiding under one of the other programs' document types (Adobe Acrobat document, PDF X-Change and Zeon). Restart Firefox, and try opening a PDF again. If it fails once more, try changing the documents again to one of the other programs, like "Use Adobe Acrobat (in Firefox)". If it STILL fails, change it to simply "Save file". Then you can simply browse your way to the download folder with Windows Explorer, and doubleclick the PDF file to see which program actually handles PDF files as default on the machine, and if it works properly. If Firefox flatly refuses to save the PDF files in a folder which you're certain both exist and has space for the file, some (probably security related) program is most likely interfering with Firefox, preventing it in storing PDF files on the system.