Please don't turn this into a Windoze SUX!!! thread. People have their reasons for using it and they deserve to be safe. It appears that is an old bug (feature?) of firefox that was reported about 9 years ago but was closed as "not a bug" because the feature is meant for people with slow internet.
As you can see from the link, it also existed in Linux but I don't have a copy installed to test. If anyone wants to check and create a Linux script, feel free to post it here and I will update the title. I never noticed the bug before because you used to be able to use --no-remote to tell firefox to only use the installation folder. When running 11.0.1 in a VM I noticed this flag seems to be ignored. Possibly with 10.x.x too.
The Bug (data leak)
When a user clicks a link to download a file, firefox has implemented a background download feature that, if the user has the "Always Ask" option enabled for where to save files, it will begin downloading while the user is browsing the "Save As" dialog. The problem is that this PART file is created in the TEMP folder under the Windows user's account. If the file being downloaded is small enough (a few MB), it will be downloaded entirely and then moved to the folder the user picks with "Save As". This is a serious data leak. If the browser or system crash before the user completes the "Save As" choice, they are left with a partial (possibly complete) file on their Windows user profile. If this is an unencrypted drive and the file is on-topic, that would be bad.
The Fix (a script)
In Windows, all envrionment variables (this includes PATH, TEMP ...) are inherited from the parent process. So we can temporarily change the TEMP path for Tor Browser without bothering the rest of the system. We just have to launch Tor Browser with a script.
1. Create a BAT file in the folder where firefox.exe is for your tor browser installation
2. Create a folder somewhere to use as an alternative TEMP folder. Create it on an encrypted drive or mounted VeraCrypt container
3. Edit the BAT file and add the following:
@ECHO OFF
CLS
SETLOCAL
REM 1. Place this script in your Tor Browser istall directory (where firefox.exe is)
REM 2. Create a folder somewhere to use as an alternative TEMP folder. Create it on an encrypted drive or mounted VeraCrypt container
REM 3. Replace the <your folder from #2> below with the full path to the folder you created
REM 4. Always start Tor Browser using this script until they fix it
REM Get current path without \
SET _thisPath=%~dp0
SET _thisPath=%_thisPath:~0,-1%
SET TEMP=<your folder from #2>
SET TMP=<your folder from #2>
START "" "%_thisPath%\firefox.exe"
ENDLOCAL
4. Make sure you replace <your folder from #2> with the actual folder
5. Save the BAT file
6. Always use the BAT file to launch your Tor Browser (make sure the folder you created is mounted and writeable)
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