Firefox (on Windows) never lost the menu bar: if you press F10 or Alt (or Alt+some letter from menu) it will pop from top. Style switcher you are referring to is located in View > Page Style, in English localization quickly accessible by Alt+V,Y. You can choose from "No style" (effectively disabling all author level styles) and "Basic Page Style" (enabled author styles). If you are lucky and stumble upon page with linked alternate style, you can still switch to it.

https://developer.mozilla.org/en-US/docs/Web/CSS/Alternative... https://developer.mozilla.org/samples/cssref/altstyles/index...

The last detail this nice feature lacks to this day and that presumably doomed it to current obsolescence is that your choice does not persist during navigation and even page reloads. Never understood why this native switcher stuck half way to be usable. Back in day alternative style sheets were (at least a bit) "hype" author had to recede to JavaScript solutions to provide persistence, so it lost much of its charm.

Speaking of ancient CSS features that looked overwhelmingly promising but never caught up as successfully as they deserved, it is probably worth mentioning User Styles. CSS was initially meant as a "dialogue" among User Agent, User, and content Author, with styles as a tool to express individual preferences. User agent sets some "browser defaults", Author sets their styles and User can participate with their own sets of preferences that can be "weak" like those defaults or even stronger than author's. It is the (or at least mayor part of) "THE 'C' for 'Cascade' in CSS".

Browsers to this days are required by specs to give user a way to participate in the cascade, but it never wasn't easy or even pleasant experience - it mostly involved editing some magic file and restarting browser (for "native" user styles) or using some extension, what feels like a poor excuse to not support it natively and mostly is implemented an a way that does not comply with specs anyway - because it mostly just spams author origin level and does not create user origin level. (I miss the old days when web was young and "userstyles" and "userscripts" seemed like natural development of tech what every user will use daily.)