One possible explanation is that Firefox 43 requires that extensions be digitally signed by the Mozilla Add-ons team, otherwise, they are disabled. (More info: Add-on signing in Firefox) Many publishers are still working through that process.

I don't know whether an update is available for the Garmin Communicator extension, but that would be the first thing to try.

Meanwhile, if you confirm that the problem is a lack of signature, you can make a temporary exception to the signing requirement. This is global for all extensions, there's currently no way to only make an exception for one specific extension.

Check Status

Open the Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then scroll to the bottom of the list on the right side where the disabled extensions are listed. Does RoboForm say it is unverified and does it lack an Enable button? In that case, this is most likely the problem.

Make Exception

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste xpin and pause while the list is filtered

(3) Double-click the xpinstall.signatures.required preference to switch it from true to false -- note that this is for Firefox 43 only

You mean this Garmin Extension https://addons.mozilla.org/en-US/firefox/addon/garmin-communicator/ ?

Garmin Communicator 4.1.0.1-signed Works with Firefox 1.5 - 43.*

Do check for updates ;)

Hmm it seems you have Garmin Communicator 4.2.0.0 which is newer? but was installed outside of addons.mozilla.org ?