You can drag this '+' button in customize mode to the far right.

• https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars

When I open the customize mode, the plus sign shows up in the position that I want (to the far right). I move it to the far left and then back to the far right and then select Done. When I get back to the normal display, the plus sign is back to its old position. Nothing has changed.

In Firefox 126, the New Tab tutton doesn't seem to want to stay at the right end unless it follows another button (that I added, not the List All Tabs button). I wonder why placing the New Tab button after the List All Tabs button no longer keeps it on the right -- perhaps there is some special logic for those two buttons.

I moved the "Recent Browsing" button as far right as i could until it was to the left of the plus sign. I did this in the customize mode. This solved my problem since I never use the "Recent Browsing" button and the plus sign stays on the far right.