Did you change the KDF? - Please read this recent thread, though it’s “mobile” related, the issue may be KDF, too…
Then in GitHub, I saw your Firefox version was not up-to-date?
And BTW, 8 chars master password is highly insecure (and the length of the master password shouldn’t be a deciding factor for unlocking / login time!). An at least 4-word-passphrase would be significantly more secure.