I've just discovered that an OpenSolaris 2009.06 guest (64-bit) no longer recognizes the back/forward mouse buttons in the browser, but my other guests do (XP, Ubuntu). These would be mouse buttons 4 & 5, or 6 & 7--I forget which are the wheel and which are the extra buttons. I've never needed to do anything to configure these on a host or a guest. They just work for all operating systems I've tried--even Solaris 10. Center button and wheel work fine. Tested with the default FF in 2009.06 (3.1b3) and FF 3.6.3 from opensolaris.org.
Host is also OpenSolaris 2009.06, 64-bit. VB 3.2.4.
I tried the LiveCD image of b134 as a guest, and it also does not recognize these buttons. I'm doing a full install of 134 as a guest so I can install the additions.
Although I didn't think it would help, I tried enabling the absolute pointing device option. Confirmed--didn't help.