Comment #4 on issue 1030 by alexander.e.popov: Ubuntu 10.04, Firefox 3.6.12: "Google Gears" could not be installed because it is not compatible with your Firefox build type (Linux_x86-gcc3)
http://code.google.com/p/gears/issues/detail?id=1030
As I mentioned above, I had this problem that I had in Ubuntu 10.04 x32 with Firefox 3.6.12. But now that I have Ubuntu 10.10 x64 with Firefox 3.6.13, I can install Google Gears without any problem (Although http://gears.google.com/ shows that 32-bit OS is required and 64-bit ones are not supported).
