Firefox looks at the TEMP an TMP environment variables for the temp folder location

• Control Panel -> System -> Advanced -> Environment variables

If you only want to change this location for Firefox then you would have to start Firefox via a .CMD (or .BAT) file to be able to set the TEMP an TMP environment variables to a different location.

As I understand, with new Firefox version, 97 or 98, Firefox now saves Temp files to Download directory. I need to change that. You say I need to use cmd? What command?

Firefox 98 no longer saves files to the Windows Temp folder when opening files in an application, like it used to do. Read How file downloads are handled in Firefox

See https://www.reddit.com/r/firefox/comments/tb6r7o/firefox_update_silently_changes_download/ Firefox update silently changes download behaviour - workaround

Related bug report (please read Bugzilla Etiquette before commenting): Bug 1738574 Want an option to continue to use /tmp and autodelete downloads automatically opened in an application Status:RESOLVED WONTFIX

The issue doesn't affect me. Changing the preference mentioned in this quote from searchfox. org may help in that it may enable you to choose your temp folder.

browser.download.useDownloadDir - bool True - Save files directly to the folder configured via the browser.download.folderList preference. False - Always ask the user where to save a file and default to browser.download.lastDir when displaying a folder picker dialog

https://support.mozilla.org/en-US/kb/about-config-editor-firefox

WARNING: Changing preferences through this interface not officially supported Hidden settings edited using the about:config tool are explicitly not supported, which means that Mozilla makes no guarantees they will be supported in the future, or that Mozilla will fix them if they break. Mozilla does not test these preferences, and will not in the future. That includes security and performance testing which these preferences may affect.

[Warning added by moderator]

As I understand, with new Firefox version, 97 or 98, Firefox now saves Temp files to Download directory. I need to change that.

Before Firefox 98, "Open with [application]" saved the file in the Windows TEMP folder, and passed that path to the application

Starting in Firefox 98, "Open with [application]" saves the file in your default download folder, and passes that path to the application

So connecting those dots, you could change your default Download folder to the system Temp folder, and Firefox should use that for content types you set to Always Ask when you select Open with [application]. In the case that you choose Save File, Firefox would display the folder browser so you could choose a different folder. Do you want to test that? Here's how the configuration looks on the Settings page in my test profile: