I use Firefox, with Chrome as backup. There's a really annoying issue that's been going on for at least a year in Firefox with regard to rendering large pages (especially dynamically-updating ones, like social media sites) if you use certain common extensions, such as Adblock Plus or NoScript (I happen to use both in my Firefox installations). For example, ABP injects stylesheets into every single iframe (which is a lot of ads, but also just completely benign stuff) loaded on a page. My computer is a little old now and can't really hold up to that so I often have to run these kinds of sites in Chrome (although I really dislike doing it, because I use Firefox for its extensibility across all kinds of tasks and have it configured in a way I can't match in Chrome) or restart Firefox frequently when it's out of virtual memory. For a long while Firefox was running really smoothly, memory leaks etc. seemed to be basically an issue of the past, but now I just have an aging desktop and am unwilling to give up either extension, so...
I used to use Opera but returned to Firefox because I don't just use my browser to, well, browse, and I have very specific preferences for how I browse because I'm also one of those people who has 100 tabs open at any given time (which is definitely another reason I should be using Chrome, since I'm making this situation worse for myself). I'd probably be willing to give Opera another go if I were actually looking for another browser, but I'm not. Dual setup seems to work fine and allow me to go about my business as usual.
I use Firefox for Android on my phone and the stock Safari installation on my iPad. I rarely browse the internet on my phone since the screen is small and I have a tablet I can tether. I don't really experience any benefits from using FF on my phone like I do cross-machine on my desktop and laptop, but none of the other browsers I tried were compelling either. iOS Safari is awful, especially for someone who runs up a lot of tabs (I do a lot of schoolwork on my iPad).