Here's a way around it:
1) Manually (ff)
- Start the developer tools (F12)
- click on inspector
- click on the arrow next to the inspector
- click on the password field (to highlight the code)
- find and remove "onpaste=false"
- Go to the about://config
- Enter "dom.events.clipboardevents.enabled"
and switch it to false
- Use the "Don't F*ck With Paste" extension (with the "u" when searching for it)