Detect when Firefox plug-in is blocked

Is there a way through JavaScript or a web service to see if a plug-in installed in Firefox is being blocked by Mozilla? In particular, on my web site, I want to be able to tell if Mozilla is blocking an outdated version of Adobe Flash and adjust our UI accordingly.

The address that Firefox uses to download the blocklist is defined as extensions.blocklist.url preference:

https://addons.mozilla.org/blocklist/3/%APP_ID%/%APP_VERSION%/%PRODUCT%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/%PING_COUNT%/%TOTAL_PING_COUNT%/%DAYS_SINCE_LAST_PING%/

Fortunately, not all of these parameters are required, this address will work to download the current blocklist:

https://addons.mozilla.org/blocklist/3/%7Bec8030f7-c20a-464f-9b0e-13a3a9e97384%7D/21.0/

This is the ID of Firefox and the version number of the current release, so the blocklist could be different for other Mozilla applications and other Firefox versions. Looking through https://addons.mozilla.org/blocked/, there are some items listed as "for Firefox version n.n.n and higher", some are listed as "for Firefox and SeaMonkey" and others again as "for all Mozilla applications". Still, it seems that the blocklist for the current Firefox version should be the most complete one.

You can download and process this file in a web service, then adjust the behavior of your web application accordingly.