Addon doesn't apply tabs.insertCSS randomly, Windows only #1134
Description
Describe the problem and steps to reproduce it:
tabs.insertCSS is randomly not applied, only in Windows 10, in both the latest FF Dev Edition or FF 70.0.1, and only when error Unchecked lastError value: Error: No matching message handler occurs and points to the line which calls tabs.insertCSS
Test URL: https://www.paypal.com/uk/signin
Please reload it many times (from 3 to 15 in my case, every time it happens differently) in order for the issue to occur.
What happened?
The sticky (originally blue) warning about cookies on the bottom of the page sometimes remains blue, while the inserted CSS should paint it red every time.
What did you expect to happen?
The sticky (originally blue) warning about cookies on the bottom of the page should always be red.
Anything else we should know?
Simple test extension is attached.
plugin.zip
This bug made many users of my "I don't care about cookies" extension to report a visible cookie warning which can't be reproduced in Firefox for Linux or even Chrome/Opera equivalent extensions which use the same code. Some of them use FF 68, so it may be a much older bug.