I went to about:config and enabled browser.taskbarTabs.enabled, as of Firefox Nightly 139, there is an Add to Taskbar option on the right-end of the URL bar.It doesn't add anything to the taskbar, but it does change the browser UI by removing tabs......
I noticed that Firefox Nightly has a (kind of) working version of this now, but it doesn't do anything other than remove the tabs from the browser lolI know this is a very early implementation, but I'd love to see how this evolves, though!
