Before the removal of the meatball menu in the address bar, users could right-click on the extension button and choose "Remove from Address Bar". This hid the button, and the extension button appeared as an entry in the meatball menu instead.
With the removal of the meatball menu (bug 1691454), the menu items to remove the button also disappeared (bug 1704474). Consequently, it is no longer possible for users to hide pageAction menu buttons of extensions. This kind of issue happened before, and caused dissatisfaction among users, see e.g. https://discourse.mozilla.org/t/add-ons-that-can-not-function-without-taking-space-from-the-address-bar-of-firefox/19044
As seen in that thread, users can work around the issue if extensions are updated to support the ability to remove the pageAction button (example). But if extensions do not get this update, then knowledgeable users can still use userChrome.css to hide specific pageAction buttons.
browserAction buttons don't have a remove button, but they can be removed via the "Customize Toolbar" user interface.