I have fond memories of Conkeror, a standalone browser built with XUL and the Firefox rendering engine. For a relatively brief moment in time, it was ahead of the competition. The absence of Firefox's baggage allowed it to launch quickly and be highly programmable.

However, over time, the importance of browser add-on stores grew, and Conkeror's strength turned into a weakness. Vimperator and similar extensions, by nature of being another extension, had compatibility with ad blockers, password managers, and other essential extensions. Meanwhile, Conkeror stagnated.

I still miss it, as it was essentially Emacs-for-web, programmable in JavaScript.