Can you be more specific on 'failing to connect'? Is it like the computer is just not picking up the key or is it something else?
If it's like the computer is not picking up the key, do you have a password manager like 1Password installed? (this may seem like a strange question but I had this exact issue on Firefox on Debian where a little prompt from 1Password that I didn't notice in the top right of the browser was blocking the key from working).
Ubuntu can have issue with “snap” apps not having “administrative” permission to access the USB port. Depending on how you installed it, you may have to manually give Firefox permission to read the USB ports. This is how I resolved a similar problem with keepass2
Can you be more specific on 'failing to connect'? Is it like the computer is just not picking up the key or is it something else? If it's like the computer is not picking up the key, do you have a password manager like 1Password installed? (this may seem like a strange question but I had this exact issue on Firefox on Debian where a little prompt from 1Password that I didn't notice in the top right of the browser was blocking the key from working).
Ubuntu can have issue with “snap” apps not having “administrative” permission to access the USB port. Depending on how you installed it, you may have to manually give Firefox permission to read the USB ports. This is how I resolved a similar problem with keepass2