source : http://kb.mozillazine.org/Unable_to_save_or_download_files
Choose a different download folder
Firefox will automatically download files to the Desktop unless you've selected a different download folder or you've set Firefox to ask where to save every file. Download problems may be related to the selected download location, for example, the download folder may contain a lot of images  or it may no longer be accessible . (To fix problems related to the last-selected download folder, you can open about:config, right-click on the preference browser.download.lastDir and select "Reset".)
To choose a different download folder, click Tools -> Options in the Firefox menu and, under "Main / Downloads" (or "Downloads / Download Folder" in Firefox 1.5 and earlier) select the "Save files to" option (if not already selected) and click the "Browse" button.
Reset download folder
Firefox may have lost track of which folder to use for the download directory. Symptoms would include "File - > Save Page As" and the "Save Image As" and "Save Link As" right-click context menu options not functioning or nothing downloading when you select the "Save" option in the Firefox "Opening" dialog box .
The solution may be as simple as choosing a different download folder, then going back and reselecting your preferred download folder in Firefox Tools -> Options under "Main / Downloads" (or "Downloads / Download Folder" in Firefox 1.5 and earlier). For example, if Always ask me where to save files is selected, choose Save files to, choose the "Desktop" folder, then go back and reselect Always ask me where to save files. 
If you still have problems you can restore the default Firefox download folder settings as follows:
Enter about:config in the address bar. A list of preferences will appear. Scroll through the list ( or type browser.download in the Filter box) and reset any of the following preferences that have a user set status (right-click on the preference and click "Reset"):
You can then change back to your preferred download folder settings, if you wish, in your Firefox Options.
If you'd rather just reset all Firefox preferences to the default values, you can delete (or rename) the prefs.js file in your Firefox profile folder or use the Safe Mode option, Reset all user preferences to Firefox defaults. See Resetting preferences for more information.
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