So here's my understanding of it.
When we're enabling Pale Moon's compatibility mode, we're sending a Firefox user agent to websites, instead of the native Pale Moon user agent. Many websites have restrictive allow lists for user agents, and using a Firefox user agent helps with these sites.
However, more and more websites are implementing feature detection in addition to user agents to determine if a browser is "lying" about its user agent. In simple terms, different browsers have different javascript engines, and the feature detection algorithms check to see if the Javascript engine of a browser is matching the User agent it is reporting.
If the JS engine of the browser doesn't match the user agent string, the feature detection algorithm flags the browser as suspicious, and a captcha is shown to the user.
Now, Pale moon shouldn't actually have any issues with this, since its JS Engine is similar enough to Firefox's (old Firefox's anyway) that it
ought to pass the feature detection test, but there are some key differences.
One example being this one:
viewtopic.php?f=5&t=31247
I'm going to spend more time hunting for these feature detection "gaps" to see if FF's compatibility mode can be made more robust.
