It's quite clearly trying to run some sort of security scan on the downloaded file and something is broken so it's waiting forever for it to finish.

You can still get to the exe file, right click the download and browse to the folder it's downloading to. Make sure you can see hidden files and also do not have known file extensions hidden (every Windows user should know how to set this) you should see the file you're downloading in there and another file with the same name but with .part on the end. Right click this "part" file and select copy, paste it somewhere else (temp folder) and rename it to what it's supposed to be. Then you should be able to run it.

Alternatively, you can also use another browser to download the same file. Right click the download in Firefox and select "Copy Download Link". Then paste this link into another browser and let that download it.

There's probably something in about:config you can change to prevent the security scan from running ( browser.download.manager.alertOnEXEOpen may be worth a try ). My guess is that some antivirus program you had installed in the past inserted something into Firefox that it didn't bother to remove when the AV software was uninstalled. This kind of thing also used to break Microsoft Office as well.

https://support.mozilla.org/en-US/kb/cant-download-or-save-files

https://www.google.com/url?q=http://kb.mozillazine.org/Browser.download.manager.scanWhenDone&sa=U&ei=uCTEVMCbMYaeyQSZh4GQCQ&ved=0CAoQFjAC&client=internal-uds-cse&usg=AFQjCNHqYhEv-s9suyMRxrYXzDDarrYjOg

NOTICE ! ! The preference browser.download.manager.scanWhenDone is no longer available in Firefox 31. Not sure when the preference was removed (I can't find a bug report) but it's still there in Firefox 24 ESR. If it matters, it's also still there in SeaMonkey 2.26.1 which is based on Firefox 29.

http://kb.mozillazine.org/Unable_to_save_or_download_files#Disable_virus_scanning_in_Firefox_preferences_-_Windows

I have the SAME problem as the OP. from the looks of it, it is unresolved. please finish this up so it can be fixed

Which version of Firefox do you have? The "owner" of this thread posted here with the ancient Firefox 27 version, which doesn't have the browser.download.manager.scanWhenDone preference.

Overall, it might be better if you started your own thread here - https://support.mozilla.org/en-US/questions/new - and provided the appropriate system details for your Firefox installation.

anything after Firefox 27 would still not have the browser.download.manager.scanWhenDone preference. I have Firefox 36.0.4 that will NOT update because EVEN the update windows in "about Firefox" hangs at 99% for quite some time before calling it done, and the SAME for all EXEs, I suspect corrupt download

Did you check the settings in your security software or possibly temporarily disable it?

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test.

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

If there are problems with updating then download the full version and delete the current Firefox program folder to do a clean install of the new version.

You can find the full version of the current current Firefox release (37.0.1) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/

Get a thumb drive. Go to another system and download the full installer into the thumb. Then plug it into your computer.