A year ago I started using them in the developer edition of Firefox on my Macbook, but then months later the Firefox developer edition started flaking out, e.g. I'd open a profile and it literally wouldn't navigate anywhere, or if it did it wouldn't last long. Bizarre. Even mentioned it to a college classmate who recently took a job at Mozilla but he's not working on anything close to that. So I started using profiles in Safari, and honestly, neither is perfect but some stuff worked a bit better in Firefox. I still go open up Firefox every now and then to retrieve a password I haven't yet migrated over to Safari. I probably should bite the bullet and get rid of Firefox developer edition and just install the regular build. I love using Firefox Focus on iOS since it feels faster than the other browsers.

edit: flaking out, since I can't think of a better way to describe what's happening :)