[Fixed] Thunderbird 5 not remembering compatable add-ons.

I suspect that the problem might be in an obscure registry entry that is saved – at least temporarily – on the host computer. I generally turn off compatibility checking in both Thunderbird and Firefox, by inserting these two lines in about:config (in Thunderbird, access it under Options –> Advanced –> Configuration...)

extensions.checkCompatibility.5.0 -- false
local_install.extensions.checkCompatibility -- false

The first entry allows installed extensions to work under Thunderbird (or Firefox) 5.0.x. You'll need different lines for Thunderbird 5.1 and so on, as well as for alpha and beta versions (ex.: 6.0a and 6.0b).
If I understand correctly, the second entry allows you to install extensions that are theoretically not installable.

To add those two lines in about:config, right-click in the about:config window and select new –> bolean value. Then type the name of the field (copy-paste from above) and the "false" value.

A warning: it will allow officially incompatible extensions to work. However, truly incompatible extensions won't work or may have undesirable effects.

I have also noticed the same problem since Thunderbird 5. Likewise its on a Dropbox installation. The installation path on both computers is c:/Dropbox/PortableApps/Thunderbird etc.

I have tried uninstalling all extensions, gone through Thunderbird's prefs.js to update or remove any old path (its an old profile so there were still many references to long uninstalled extensions, older installation paths etc).

I have also done a little exploring and come across something a little strange, or at least something I don't understand.

I have seven extensions. Examining the install.rdf of every extension reveals that of these, only three include an 'em:maxVersion' up to or beyond 5. Unsurprisingly, the 4 that have an 'em:maxVersion' less than 5.* are the extensions that report as incompatible when switching computers.

As with the others above, telling TB to update the extensions (or downloading the latest version) does not change this value, but it does allow the extensions to operate. I guessed maybe TB keeps something like a "whitelist" of extensions that it allows to operate beyond their maxversion but I could not find anything like this.

I thought I was onto something when I searched for the emid of the extensions and found entries in both TB's blacklists.... but then I saw that all extensions used the same emid, including those that always work, plus a handful of other IDs.

The extensions in question are as follows:
GB Dictionary
Attachment Extractor
Folder Account
Extra Folder Columns

They are all latest versions, download from Mozilla and apparently compatible upto TB 8.0!

I have also tried the obvious and amended the install.rdf of one of the offending extensions with a maxVersion of 5.*

Amusingly after restarting TB (on the same PC) the extension with the amended install.rdf now reported incompatible again on both PCs (at least until I told TB to update the extensions).

So clearly in the case of these extensions, the maxVersion is not defining compatibility with TB. This is happening somewhere else but I don't know where. What's more, wherever this is being written is not transferring between PCs. As I am using Dropbox to sync these profiles its possible (though unlikely) that Dropbox is not picking up the change. It will be worth copying the installation onto a USB stick to replicate and reconfirm but based on the experience of the OP I expect the same result.

At a guess TB is caching or temporarily listing compatible extensions, and is clearing this cache when it detects it is running on a new PC. That's my guess anyway...

Unless anyone has any ideas I suppose the next step would be to contact the authors of the extensions, or Mozilla.