In my previous work at a company that made it's money from a browser extension, I saw what hell it was to develop browser extensions for all major browsers.

I should note it looks like this issue is mostly fixed going forward. The chrome extension model won, and Firefox and Windows Edge are supposed to be supporting either directly or through electrolysis.

I.E. was not an extension, but a toolbar, written in C++ (being C++ the debugger was Windows visual studios so it was doable). Firefox was Xul based and almost impossible to debug at the time (much improved in recent months), and chrome just worked.