For a new setup, I tried today to install Lightning for Thunderbird under an Ubuntu 11.04 64bit over the addon mechanism which failed with an ominous message about my build platform not being supported. This is due to an over-precautions setting in the plugin’s installation manifest which can be changed easily similar to changing version mismatches.
The short story: adjust in the install.rdf
The complete story:
Here is the way to fix this:
1. When installing the plugin, the error message will say what exactly your build platform is. Note that carefully down, for my setup it was Linux_x86_64-gcc3
2. Download the Lightning addon. Mine was called lightning-1.0b2-tb-linux.xpi
3. Rename it to lightning-1.0b2-tb-linux.zip and edit the install.rdf file
4. Locate the entry
5. Replace something with the platform you noted in 1
6. Save the changed install.rdf and change the zip file suffix back to xpi
7. Install the addon
George's Techblog 
good tip.
my version had for both platforms. I removed the x86 version and it worked instantly.
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What i mean by both platforms is to seperate platform statments
Linux_x86_64-gcc3
and Linux_x86-gcc3
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