Bookmarklets are a great way to add functionality without writing a full-blown browser extension. They should work in all the major browsers (even Opera which I love). I have created simple bookmarklets to adjust screensize. It works when only one tab is open, but I can check the responsiveness of my design quickly.
An economics podcast is an unusual place for a discussion of UI design, but this conversation with Eric Raymond (The Cathedral & the Bazaar) includes the usability of open source software. Open source development works when, Capital goods required to do the work are cheap. The limiting factor on the work is human creativity and attention. The work is intrinsically rewarding. There is an objective metric for success that everybody in the hive mind can agree on without too much effort. Without that condition you get thrashing. Zero-cost communications.