Too Many Tabs
I switch between windows with Super-J/K, or Super-C to switch within a frame. Firefox: Ctrl-Tab. I can also use Shift-J/K since I use tridactyl. Vim: gt/gT to switch tabs, :bn and :bp to switch buffers. Tmux uses Ctrl-b+n/p to switch tabs. Weechat uses Ctrl-n/p to switch chat buffers. There's probably another combo for emacs, but I'm not that advanced of a user yet.
I often find myself using the wrong modifiers to switch tabs in a program, for some reason. Compounded by my new keyboard, on which I've bound space as a mod-tap Control key. So the bindings change if I'm at my desk or using my laptop.