From MozillaZine Knowledge Base
This preference allows disabling of extension compatibility checking. This is useful if you are using alpha, beta or nightly (development) builds which Add-on authors do not yet support. However, enabling incompatible extensions may cause crashes or other malfunctions.
Note: Many extensions may have updated versions not on their main addons.mozilla.org page, so at the bottom of such extension's pages use the link for "View All Versions" to see if there is an updated version available, or to be able to install an older or experimental version.
Note: In Mozilla applications based on Gecko 1.9.2 and above (e.g., Firefox 3.6 and SeaMonkey 2.1) this preference is no longer used and a new preference that includes the application version, extensions.checkCompatibility.<version> is used instead (bug 521905).
"<version>" includes alpha and beta milestones but excludes minor version updates. It is the first few characters of the application version matching the following pattern:
A few examples are in order:
Possible values and their effects
Check extension compatibility on install and update. (Default)
Do not check compatibility. Incompatible add-ons will be marked in the Add-ons Manager - its icon will have an exclamation mark over it and when the add-on is selected, the text Not compatible with <application and version> appears below the extension description.
If disabling extension compatibility checking makes your application crash on startup, you should try starting in Safe Mode and re-enabling compatibility checking.
Mozilla Firefox 2.0 through 3.6
In versions since 2.0 and up through Mozilla Firefox 3.6.x, this preference can be toggled to true by clicking the "Enable" button in the Add-ons Manager next to the “compatibility checking disabled” alert. (There is no way to set it back to false within the UI)
Note: The UI has been removed in Mozilla Firefox builds after 3.6 (since at least Firefox 3.7a5).
Same as above.
First checked in
Has an effect in