Generally, there is not enough space to show the whole page title on the tab. The URL bar, of which the complete URL is not cared about by the most users, is a good place to show it.
In the old days, I have made an extension for that purpose:
Now I created a userChrome.js script version for it:
https://github.com/Merci-chao/userChrome.js?tab=readme-ov-file#page-title-in-url-bar
It would be great if it is a native optional feature in Firefox. I remember that the Android version had it, but has been dropped now.