It was not removed. Are you using custom userChrome.css modifications? Try to disable it in 'about:config' by switching toolkit.legacyUserProfileCustomizations.stylesheets to false then restarting the browser.

Go into Customize mode and drag it back into the toolbar or click the "Restore Defaults" button in the bottom-right corner.

I tried that already, but it destroyed the UI: the specific file I use is intended to put the tabs bar below the address bar, directly above the page, where it's supposed to be. is there any way to get the dropdown back while keeping my tabs bar exactly where it's supposed to be?

You will need to update your userChrome.css, see if the original author has any updates available or ask for help in one of the CSS communities.

like, look at this; the tabs bar should be on top of the page, not on top of the address bar. this is disgusting and hideous and it was fixed before this last update.

the only reason I even resorted to userChrome.css in the first place was because some genius removed the "tabsontop" option from the about:config menu.

so new problem: how do I get the tabs bar back where it's supposed to be??