The section linked below is supposed to allow adding a file type. Good luck.

https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file#w_adding-download-actions

Thanks for prompt, but it didn't help much... Still, the needed content type is not in the applications list and it seems like I cannot add it manually.

Just to note, if I didn't properly, the compressed archives formats (zip, rar, 7z etc. etc.) are that I actually wish to be opened (as temporary), not saved permanently (like, into Downloads folder).

I haven't had to deal with the issue. Changing the preference mentioned in this quote from searchfox. org may help.

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]

I haven't had to deal with the issue. Changing the preference mentioned in this quote from searchfox. org may help. 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]

Tweak worked, but the main problem persists. After the option appliance the browser simply saves whatever file automatically without asking whether to open it (into the Temp folder) or Save wherever thefore acting Chrome alike. I'm steel in need of this FF little (but significant) behavior prior to 98.0

You could try changing this preference to false but it's a temporary fix (if it works at all). browser.download.improvements_to_download_panel https://support.mozilla.org/en-US/questions/1370494

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]