To resolve this error:
“Google Gears” could not be installed because it is not compatible with your Firefox build type (Linux_x86_64-gcc3). Please contact the author of this item about the problem
Stan Joel posted the lastest (5.18.0) on Issue 335
Download: http://jms.id.au/~shenki/gears/gears-linux-x86_64-opt-0.5.18.0.xpi
In Firefox, click File -> Open File, find gears-linux-opt-0.5.18.0.xpi
Click “Install Now” when prompted
Restart Firefox
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