Melllvar

Looks like compatibility hacks for various websites.

Interventions - are deeper modifications to make sites compatible. Firefox may modify certain code used on these sites to enforce compatibility. Each compatibility modification links to the bug on Bugzilla@Mozilla; click on the link to look up information about the underlying issue.

User Agent Override - change the user agent of Firefox when connections to certain sites are made.

https://wiki.mozilla.org/Compatibility/UA_Override_&_Interventions_Testing