Please send a screenshot of the problem you are having.

Can you see the whole window in safe/troubleshoot mode (hold Shift when launching TB)?

Regret, no, safe mode did not help. The lowest past of the Filter Rules screen is invisible and cannot be revealed. Bug ID 1879791 dated 5th Nov 2024 matches my problem. I am running TB 128.6.1 esr (64-bit) This bug appeared to me recently – date / TB version unknown. Previously not a problem. My screen res is 1920 x 1080

I've seen reports of this in the past few years, but never experienced it myself. The filters window can be resized by clicking and dragging the corners, and the new size should be retained. Did you try the userChrome.css method shown in the Bugzilla report, as a last resort? If you haven't used css before, reply here for a guide.

Clicking the corners of the Filter Rules window simply moves the window down by about 12% in the display, thus further hiding the lowest part of the window. There is no drag functionality obtained. I have not used the userChrome.css method that you suggest. I would be grateful for the guide that you offer.

On my setup, I hover the cursor on the corner of a window until it becomes a double arrow, then click and drag the window to resize it. Do you see the double-arrow cursor in safe mode?

To apply the css code, Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files *.*, copy in the Bugzilla css code, change the numbers as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config. editor, restart TB.

Online css generator

video on how to create a css file (Firefox and TB)