HttpWatch added support for Firefox 2 & 3 back in 2008 and we’ve updated it to support the 39 major versions released since then.
Sadly, the last Firefox version HttpWatch will support is 40 (the one released this week, August 11, 2015). The main reason is that Firefox 41 will drop support for native extensions like HttpWatch:
Unfortunately, porting the HttpWatch extension to JS/XUL or using JS c-types is not feasible due to the large development effort required, the inability to maintain a low performance overhead and the limited interfaces available to the underlying Firefox web and network components.
It’s not just the dropping of binary extension support that has forced this decision. The rapid 6 week release cycle has made it increasingly difficult for developers and Firefox users to keep their extensions working with each new release. We have come to the conclusion our development time would be better spent adding more features to HttpWatch on Internet Explorer and adding new ways to sniff HTTP traffic from other browsers.
If you want to carry on using HttpWatch, for as long as possible, the best option is to use Firefox ESR version 38. It will work with HttpWatch version 10 and receive security fixes until approximately March 2016.
UPDATE: Mozilla is also deprecating the XPCOM and XUL technologies that have been the foundation of the Firefox extension system.
UPDATE: You can still Firefox 40 to test HTTP/2 with HttpWatch by downloading it from https://ftp.mozilla.org/pub/firefox/releases/40.0.3/win32/en-US/