Mostly that FF has ~36k extensions (January 2024) [1] and Chrome has ~112k (June 2024) [2].

[No doubt, total count of extensions isn't the most important number and there's a long tail in both counts of very small user bases, but this paints the ~3x picture in a broad stroke.]

Of course, since FF migrated to WebExtensions in 2017, theoretically most Chrome extensions can be ported to FF with minimal changes [3] — practically speaking though, not all of the big ones actually have, or the FF equivalents to some of the most useful Chrome extensions are far less polished.

And also, if you're developing front end web apps for normal end users, most are still on Chrome... over the years, I've experienced an unfortunate number of sites that should work across Chrome/FF/Safari actually break because of things like the developers not even testing in browsers besides Chrome given its dominance. I'm not encouraging that by any means, but the reality is that it still happens.

[1]: https://en.wikipedia.org/wiki/Add-on_(Mozilla)

[2]: https://github.com/DebugBear/chrome-extension-list

[3]: https://extensionworkshop.com/documentation/develop/porting-...

Visually Chrome now look more like Firefox, really, thin tabs with declining borders have been substituted by thicker one with vertical border. Practically if feel faster on the same iron, and it's more rigid in user UI change, for instance you can't have Tab Center reborn and hide the tab-bar because you are on a large screen and you like to been able to read comfy all tabs...

For the rest... They are essentially equivalent even if under the hood there are many differences.

Nothing, other than you can't debug JavaScript as easily as with Chrome or Node.

Mostly sites complaining they don't recognize your browser and disabling features (this is mostly a blessing).