[gshdg]

History.

Firebug was the best and nearly only tool out there for a while.

Then Google took a page from Apple’s early OS X book: bundle great developer tools so developers want to write apps on your platform and not port them to others.

Meanwhile Firebug was suffering and non-performant due to not having access to the browser core. Chrome’s developer tools soon outstripped it.

Mozilla decided to rebuild Firebug’s features internally where they’d have the right permissions to perform well and debug properly and do all the fancy things Chrome’s did.

Developers loathed Internet Explorer but had to develop for it anyway, and I assume Microsoft added tools to it to keep their browser from being blackballed entirely.

Safari? Who knows.