You can check for issues caused by plugins and set all plugins to "Ask to Activate" on the "Firefox/Tools > Add-ons > Plugins" page (plugins are not affected by Firefox Safe Mode)

Boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

Try to delete the MRU (Most Recently Used) ComDlg32 registry keys with the registry editor (regedit.exe). Navigate to: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\ComDlg32 Delete the subkeys (LastVisitedPidlMRU and OpenSavePidlMRU) Be cautious with editing the registry as there is no UNDO possible: all changes are applied immediately. You can export registry keys before making changes.

Hi cor-el Thanks.. : 'Ask to activate' was already enabled (did that a wek or so ago, I remember now seeing that suggestion). Windows safe mode tried; registry subkeys deleted .. no improvement..

Do a clean reinstall and delete the Firefox program folder before (re)installing a fresh copy of the current Firefox release.

• Download the full Firefox installer and save the file to the desktop
  https://www.mozilla.org/en-US/firefox/all.html

If possible uninstall your current Firefox version to cleanup the Windows registry and settings in security software.

• Do NOT remove "personal data" when you uninstall your current Firefox version, because this will remove all profile folders and you lose personal data like bookmarks and passwords including data in profiles created by other Firefox versions.

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• (32 bit Windows) "C:\Program Files\Mozilla Firefox\"
• (64 bit Windows) "C:\Program Files (x86)\Mozilla Firefox\"

• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.
• http://kb.mozillazine.org/Uninstalling_Firefox

Your bookmarks and other personal data are stored in the Firefox profile folder and won't be affected by an uninstall and (re)install, but make sure that "remove personal data" is NOT selected when you uninstall Firefox.

If you keep having problems then also create a new profile.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

SOLUTION (your mileage may vary) -- originally posted on a similar post here: https://support.mozilla.org/en-US/questions/991703#answer-618728

I was experiencing precisely the same behavior.

Then, based on a comment by user "mattyjt" over at BleepingComputer (http://www.bleepingcomputer.com/forums/t/505204/file-upload-window-hangs/), I disabled Bluetooth in the Startup tab in Task Manager, then stopped the three Bluetooth services in the Services tab (later, I followed this up with open Services.msc and setting all three to Manual).

That did the trick. Immediately (I didn't even have to reboot), I could use Firefox's file dialog again. Don't know why. But, reasoning out how file dialogs are typically called in programming, and seeing how your issue was resolved by disabling startup items, I began to suspect that it wasn't a Firefox issue, per se, so I began to google around without the word "Firefox", which led me to the BleepingComputer forum linked above. Hope this works for you as it did for me!