Actually, you need an add-on for this. For example:

Windows Titler: https://addons.mozilla.org/firefox/addon/window-titler/

Thanks. I've never added any add-ons to Firefox. Are they safe and reliable in your opinion?

Just read the user reviews for their Addons to see what user experiences are then you can make your decision from that.

I agree with reading the reviews. Also, we have an article on how to assess the safety of an add-on:

Tips for assessing the safety of an extension

Since Window Titler hardly does anything, it doesn't need intrusive permissions like seeing page content. If it is ever updated to need more invasive permissions, Firefox won't install the update without your approval.

Two other considerations:

(1) Not updated since July 2019, so no risk of recent shenanigans

(2) Over 3,600 users

I took a look at the reviews, but there was only a small number of reviews in proportion to users, so not representative. But maybe that's the way reviews go on add-ons?

The tips is a good link too.

Many thanks - I'll go ahead and load Window Titler.

So I have installed Titler from the add ons page, but can't see it the button on the toolbar anywhere. I followed the instructions to customise my toolbar to add it but it doesn't appear anywhere in "more tools" for me to select it to add its button to the toolbar. Also I followed instructions to use Ctrl-Alt-T as a shortcut to invoke it, but nothing happens. I have added the default title bar from settings as another step in its instructions, and that worked fine. Now I just want to actually use Titler, but it's invisible! Where is it hiding?

Also bizarrely since I installed it I cannot select anything from Firefox's main menu except for Firefox (so not File, Edit, View, History, Bookmarks, Tools, Window or Help)!

Use the Extensions button to access your extensions. If you don't see it, click the "Restore Defaults" button in Customize mode.

I followed the instructions to customise my toolbar ... Also bizarrely since I installed it I cannot select anything from Firefox's main menu

The menu bar is disabled in Customize mode.

So I have installed Titler from the add ons page, but can't see it the button on the toolbar anywhere. I followed the instructions to customise my toolbar to add it but it doesn't appear anywhere in "more tools" for me to select it to add its button to the toolbar.

As mentioned by zeroknight, Firefox recently started consolidating add-on buttons under the Extensions button (looks like a puzzle piece). You should be able to drop that list and pin the Window Titler button to the main part of the toolbar.

Also I followed instructions to use Ctrl-Alt-T as a shortcut to invoke it, but nothing happens.

Hmm, you can double-check that shortcut using the Extensions Shortcuts setup page: Manage extension shortcuts in Firefox.

Also bizarrely since I installed it I cannot select anything from Firefox's main menu except for Firefox (so not File, Edit, View, History, Bookmarks, Tools, Window or Help)!

I don't know why that would be. (As a Windows person, we have a different menu setup, so I can't mimic your situation.)

Use the Extensions button to access your extensions. Got it. And with the other answer I have successfully pinned it to the toolbar.

l569 said

I followed the instructions to customise my toolbar ... Also bizarrely since I installed it I cannot select anything from Firefox's main menu

The menu bar is disabled in Customize mode.

Only stopped working when I installed the add-on which didn't involve any manual disabling of it. Can't see any mention of these functions and the overall Firefox menu bar in the article you highlighted, which part are you referring to and where do I un-disable the menu bar? I do see the hamburger menu drop down from the toolbar and that works OK, but I want my overhead menu options as described to work again too.

I tried to give you a thumbs up by the way too but it won't let me!