I can reliably reproduce the issue in a 18.04 VM with firefox 63.0. Unsetting the GTK_IM_MODULE environment variable makes the issue go away (see also bug #1765304, which is probably related).
The same version of firefox in Ubuntu 18.10 is not affected, so it appears to be a bionic-only issue (which is consistent with what has been observed for similar issues related to firefox+ibus+password fields).
I can reliably reproduce the issue in a 18.04 VM with firefox 63.0. Unsetting the GTK_IM_MODULE environment variable makes the issue go away (see also bug #1765304, which is probably related).
The same version of firefox in Ubuntu 18.10 is not affected, so it appears to be a bionic-only issue (which is consistent with what has been observed for similar issues related to firefox+ ibus+password fields).