It's reasonably well known that Firefox addons in Firefox Quantum (ie, WebExtensions addons) are more limited than pre-Quantum addons were. One of these limitations is the places where addons work at all. Some addons are not deeply affected by these limitations, but ones that deeply modify Firefox's UI, such as a gestures addon or an addon that adds a Vim style interface (via) are strongly affected because the limitations restriction where they can be used and thus where the UI works as you expect. In other words, where gestures work for me.

One limitation is explained directly in Foxy Gesture's Github README, so I'll just quote it:

More importantly, the mouse gestures will not work until the document body of the website you are visiting has parsed. In other words, the DOM must be at least partially parsed but content does not have to be loaded. [...] This is an inherent limitation of web extensions at the moment, because there is no API to get mouse events from browser chrome. In practice this is rarely an issue as mouse events are typically available very quickly.

This is almost always true in practice, because Firefox Quantum loads web pages very fast. Well, it loads them very fast when their web site is responding. When their web site isn't really responding, when you're sitting there with a blank page as Firefox tries to load things and you decide that you're going to give up and close the tab, then you run into this issue. I close most tabs through a mouse gesture, or at least I would like to, but when a new tab hangs during the initial page load (or sometimes during subsequent ones), my mouse gesture doesn't work and I have to turn to Ctrl-W on the keyboard or clicking the appropriate tab control.

The other big limitation of addons is that they can't act on pages that Firefox considers sensitive pages, especially including internal chrome pages. Unfortunately it turns out that a number of pages that you wouldn't expect are considered chrome pages, and these are pages that you may use all the time. Specifically, pages in Firefox's Reader mode are all considered chrome pages and off limits to addons, as are all pages that are showing PDFs using Firefox's internal PDF viewer. The Reader mode limitation is especially irritating and makes Reader mode quite a bit less attractive to me; if you're going to break my UI and not always work, I wonder what I'm really getting out of it.

(With both Reader mode and PDFs, there's no indication in the displayed URL itself that you're in some special internal Firefox chrome page context, since they display the normal URL. This is especially striking and irritating in Reader mode, at least to me.)

Two more important cases of chrome pages are Firefox's network errors page (what you get if you leave one of those slow-loading web pages to actually time out) and about:blank, the completely blank page that shows up under some circumstances. For instance, if you open a URL in a new window or tab except that Firefox decides that the URL should be downloaded instead of shown, you're left with an about:blank page.

(A small but irritating additional case is 'view source', which is of course another internal chrome page these days.)

I'm sure that Firefox has good internal reasons for preventing addons from injecting things into these pages, but the resulting UI glitches (where gestures suddenly stop working on some page and I have to remember that oh yeah, it's a PDF or whatever) are reasonably painful. I really wish there was some way to tell Firefox that no, really, I actually do trust Foxy Gestures that much.

(The gestures that I would like to use on all pages include general window functions like 'close tab' and 'iconify'; on PDFs, I would also like things like 'increase/decrease font size'. None of these are specific to HTML content, and the window manipulation ones are basically global.)