I'm still eagerly waiting for per tab volume control [0], a per tab activity monitor/profiler [1], the possibility to suspend tabs with practically no memory usage [2] and more options for searching the browsing history [3]. Besides that I find it counterintuitive that a revisit of an URL removes the entry of the previous visit. The result is an incomplete history. I wished they would change that.
I know it isn't necessarily the same, but volume controllers on Linux using Pulseaudio have per-source volume meters. I use them to turn off annoying banners and such.
I don't think tab volume control is trivial; NPAPI doesn't provide an audio API. So Flash on Win32 (for example) is just sending audio directly to the OS. (You can use the Windows volume controller to adjust Flash volume, at least.)
Technically, they could set LD_LIBRARY_PATH (or equivalent in Windows) and provide a sound API wrapper library that could control the volume. That sounds overkill, though.
[0]: https://bugzilla.mozilla.org/show_bug.cgi?id=728046
[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=400120
[2]: https://bugzilla.mozilla.org/show_bug.cgi?id=675539
There are add-ons for suspending tabs but in my experience not very robust ones
[3]: Full text search and search operators for time intervals would be great.