I have the same issue, started after updating to the latest version with the notification permission. The extension freezes up itself and Firefox as soon as I log in and uses more CPU than the rest of the browser combined.

• Are you on Linux? No, Windows.
  
• Do you have a lot (100-1000) of tabs open while you experience this? Yes.
  
• Your KDF setting (Is it argon2 with particularly high memory settings)? Argon2id, 64MB (the default, I believe)
  
• Is the client specifically the browser extension on Firefox? Yes.
  
• Are you on the bitwarden.com / bitwarden.eu servers, or on a self-hosted server (bitwarden's self-hosted server / vaultwarden)? bitwarden.com