Thanks Asyn, I've followed the instructions for Malwarebytes and have excluded the Avast files and vice versa.
Repairing Avast was one of the first things I tried before I reinstalled it but it didn't work. Have tried it again anyway and despite excluding Malwarebytes it flagged it as incompatible.
Still no email.
Update: I retrieved my passwords and the extension is working! I followed these instructions found in the FAQs of the Passwords board
If the Avast Passwords browser extension reports an issue:
Go to your internet browser Extensions section and uninstall the Avast Passwords browser extension. For instructions on how to uninstall extensions, refer to your internet browser's documentation.
Open the Avast user interface and select Privacy ▸ Passwords.
Click Settings in the top-right corner of the screen.
Select the Browser Integration tab from the left-side panel and click Add extension next to the internet browser reporting an issue