Oh gods, they fucked up Firefox, WTF is happening, I can't tell one tab from another anymore
— Trishank Karthik Kuppusamy (@trishankkarthik) June 1, 2021
You used to be able to change how tabs look too, so they aren’t so obscured, but no longer. Fake security. The real reason has of course nothing to do with security, but rather to herd people in certain ad-friendly and control-friendly directions. Also, it’s to de-emphasize tabs, claim people “don’t use them anymore” and then force search only on everyone.
Mozilla has been pulling this trick for years, and it had worked. No reason for them to stop now.