I am not aware of a relevant preference in about:config.

When I inspect the drop-down items using the Browser Toolbox, there's no URL to reveal using custom style rules.

Depending on your needs, you could consider an add-on approach. For example, mine allows you to reassign the toolbar button to show a list of the 15 most recently accessed tabs in the window, and the URL is listed:

https://addons.mozilla.org/firefox/addon/switch-to-previous-active-tab/

You could move the button to the Tabs bar using Customize. (Sorry for the ugliness -- I actually got a bad review for the button artwork!)

Edit: Version 1.6 has a button icon with transparency that is less ugly on the Tabs bar. Also, it has an option to show the URL only on hover to make the list more compact. See: https://addons.cdn.mozilla.net/user-media/previews/full/212/212656.png?modified=1545539336

I'm not seeing them either. This still works in Firefox 60 ESR and in Firefox 61, but not in 62 later including the current Nightly build (66).

Possibly caused by this redesign.

• Bug 1446101 - Convert all tabs popup to photon panel

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

jscher2000 said

I am not aware of a relevant preference in about:config. When I inspect the drop-down items using the Browser Toolbox, there's no URL to reveal using custom style rules. Depending on your needs, you could consider an add-on approach...

Thanks, but I zip up and down the Tab List and need the pop-up URLs like in 61.x.

~Pj

You could file a bug requesting that it be added. I searched for list all tabs and didn't see that yet:

https://bugzilla.mozilla.org/buglist.cgi?quicksearch=list%20all%20tabs&list_id=14486164

(I sorted by "changed" and skimmed down the titles for 2018-2017)