Do frontend developers… not check whether their website works in Firefox?

Oh, we check. Of course we check.

And then we find that it doesn’t work, because Firefox has some weird bug. Some really dumb bug, that makes Firefox render a page in a different way than every other browser. Some bug that has existed for 20 years (!!!) and is still unsolved. (Or 15 years. Or 17 years. Or 9 years.)

Or Firefox doesn’t support some feature that every other browser supports. Or Firefox’s mobile simulator does not properly simulate a touch device. Or Firefox fires bizarre, superfluous events on mouse wheel scrolls. Or Firefox can’t discriminate between the left and right mouse button. Or Firefox has stopped supporting the ability to implement special CSS workarounds for Firefox. Et cetera, et cetera, et cetera.

And then what? Well, “just” fix the problem, right? “Just” put in additional development time, to compensate for the fact that Mozilla’s development process is increasingly dysfunctional? “Just” devote 500% more effort, to support… [checks notes]…

… 5% more users.

Hm.

Is Firefox so niche these days?

Yes.

I am (and any good web developer is) sympathetic to the need to support various minorities of users (by browser, by OS, by device, by physiological capabilities, by network limitations, etc.). I can tell you that I make every reasonable effort to support not only Firefox, but every browser I hear about. (I’ve responded to user complaints about lynx, links, qutebrowser, Dillo, and TenFourFox, to take several examples of varying obscurity; and in each of the cases I have in mind, I was able to solve the issue.)

But the fact is that fully supporting Firefox takes up an utterly disproportionate amount of effort. For developers who do not have the budget of a megacorporation behind them, that amount of effort cannot be justified. Many of us try. Most of us do not fully succeed. Some give up. Can you blame them?

No, it works on Firefox but there's a menu option to deactivate the widget and you likely deactivated it when you aren't seeing it.

It shows up for me. Here is a screenshot from my Firefox:

You can see it in the bottom right corner.

I do indeed test our website on Firefox about once a week and make sure everything works, and also do a bunch of crash-tracking to understand whether there are any browser-related issues that show up. You might be using an outdated version of Firefox, and sadly the development effort for testing all current versions of Firefox (and it's not just about older versions, most of our browser-related bugs actually come from more recent releases that then get patched) is beyond our development capacity.