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Slows down website loading significantly and overall Firefox performance is also decreased since a while.

This is what happens when a market leader gets complacent. Metamask is "okay", but everyone uses it because so many Web3 sites support it by default. The reality is, considering their marketshare and revenue, they should be investing in building out proper multi-chain support, NFTs, keyboard shortcuts, and even just basic stuff like having the icons for tokens render correctly. Even important things like the gas price "suggestions" seem to be always wrong, and charge more than necessary while also getting transactions to fail (if you don't believe me, check gas on ethgasstation and enter the values from there -- it will go through, and it will for sure be much cheaper, even though metamask will give you a warning that it's likely to fail).

Pro: access to great Defi products.
Con: google password sniffed/hacked a few hours after install, possibly originating from another malicious tab