Customise auto generated passwords

Hi. Usually any passwords auto generated by Firefox used uppercase, lowercase, and numbers - no symbols. As of very recently, all passwords being generated now include special characters. I didn't change this so I assume it came with an update. Ordinarily this would be fine, however the characters being inserted are often invalid ones and I've found a few websites unable to accept passwords including them, such as < and : making the generated password useless. Is there a way to customise how passwords are generated? I've looked through settings but can't see any way to adjust it. I'd prefer to remove special characters completely. Thanks!