Actually it takes more as there are other Mozilla folders in other places but ya 121 megs is what my Mozilla Firefox folders size is. Seamonkey browser folder is 89megs Large differences in coding for using all cores of your cpu instead of just 1 core. I would expect more or less for each version as that is what happens with progress for speed, security, privacy, mobility. Firefox 57 Quantum is to be released Nov 14th https://www.mozilla.org/en-US/firefox/quantum/ https://www.theregister.co.uk/2017/09/26/mozillas_new_firefox_quantum/ No spyware

Please let us know if this solved your issue or if need further assistance.

So version 56.0 of Firefox for Windows does take up over three times as much space as version 55.0.3?

MrHh said

So version 56.0 of Firefox for Windows does take up over three times as much space as version 55.0.3?

NO, the 100Mb difference is the Profile folder contents in "Users" both Local and Roaming foldersets. Due to those foldersets being shared by both versions Windows only counts it once and attributes those folders to the newest installed version.

But unless you purposely installed Firefox 56.0.1 to be a 2nd installed version, in a separate Program Files folder, you probably only have the newer version installed; and the Windows Registry "choked" during an update or installation and is reporting both versions as being "installed".

One way to determine that is to "Search for Files or Folders ..." for firefox.exe and see how how many are installed; then take note of the folder location of each to verify their existence.

Or, uninstall the older version of Firefox thru Add or Remove Programs via the Windows Control Panel. Then when Firefox won't launch, because it's been uninstalled, you can re-install the latest version and the Windows Registry fault should be corrected.