I switched to Vivaldi a year or two ago after using Firefox as my main browser for the longest time. I've considered switching back, but every time I run up against several features in Vivaldi that I am just not ready to part with yet:

1) tab stacks, 2) the ability to save and reopen an entire session, 3) a page load progress indicator in the URL bar, 4) using Ctrl-F highlights the position of all matching locations in the page in the right-hand scroll bar, and 5) the window panel, which keeps track of all closed tabs and windows so you can reopen everything if you accidentally close a window.

Some of those are small perks, and some of them are major. I'd love to see Firefox implement some of them.