When all the adblocking news came out I switched to Firefox. Used it for two weeks exclusively, then I switched back. There are a lot of things that just don't work, or work in a weird way. For example, I use Linux with the Arc Dark theme, and Firefox shows the inputs, textareas and buttons dark background and dark text, which results in an almost unusable experience on a lot of websites. Of course, this can be fixed, if you add an extra config (so not editing an existing one), and you set the "inner" styles to use the light Arc style.
Next, my laptop has a touch screen, and I cannot use it to scroll. When I'm listening to music on YouTube, even if it is in the background, it uses so much CPU that my whole laptop becomes extremely hot. Also, I'm developing a website where there is an element that has overflow:auto on it, and has some inner paddings, Firefox doesn't render the bottom one. The buttons are rendered with the text off center on all Bootstrap 3 websites, and for example on Github a bunch of buttons, badges, labels, and pagination elements have their text vertically off center too.
I know that all of these could be happenning, because we (developers) optimize things for Chrome now, but when the CSS styling works as expected in Chrome, and not in Firefox, then it certainly feels like Firefox has the bugs, because Chrome does what I expect it to do.
Lastly, on my iPhone, the browsing experience was really bad. Frequent freezes, slow page loads, when I closed a tab, the whole tab screen scrolled to unexpected positions.
All in all, the small things are the ones that ruin the experience for me, and Firefox has a lot of them (at least for me).