Today I discovered that someone has patched Gears to work with Firefox 64-bit, and released a precompiled installer. Warning: after installation, when Firefox restarts, you will see a warning that the plugin could not be installed (‘”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.’). However, it is installed and works just fine. Unfortunately this message is repeated each time Firefox restarts.
I can now browse my admin page with Firefox in offline mode. Sniffing proves that not a byte is passing. Now I can write blog posts on planes, without having to use an offline text editor.
Update 2009-05-29: My old colleague from my days at Greenpeace, Niels Peen, now provides the latest version (5.21.0) with proper build tagging. So you will no longer see the warning about Linux_x86_64-gcc3 being the wrong build type. Thanks, Niels!
Update 2009-07-24: 5.31.0 and 5.32.0 are now available here, also with build instructions so you can do it yourself!