Doing a quick Google on this behavior led me to a post that this happens when an add-on thinks it's compatible with the current version of Firefox. To fix this, I needed to run Firefox in "Safe Mode" and disable all add-ons.
The command to start in "Safe Mode" is:
$ firefox -safe-mode
And you will be greeted with a dialog box where you can tick the checkbox to disable all add-ons.
Once I restarted, Firefox worked fine.
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Firefox (ESR) version as of this post: 10.0.10
Adblock Plus version as of this post: 2.2