Reproduced this issue on the latest Nightly version (2020-11-19) using a very long email address used to create and sign into Firefox Accounts on a macOS 10.15 device. This seems to be an old regression. Mozregression revealed the following pushlog: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=9136070a0561cb169b01569b2346a1e3aad387dd&tochange=2b8364cfdb04bfdfde7c01589e48e24973c1ce9c From which, I suspect Bug 1575706 to be the regressor of this issue.
Bug 1676759 Comment 5 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Reproduced this issue on the latest Nightly version (2020-11-19) using a very long email address used to create and sign into Firefox Accounts on a macOS 10.15 device. This seems to be an old regression. Mozregression revealed the following pushlog: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=9136070a0561cb169b01569b2346a1e3aad387dd&tochange=2b8364cfdb04bfdfde7c01589e48e24973c1ce9c From which, I suspect Bug 1575706 to be the regressor of this issue. Suggesting S3-S4 severity since this requires a really long mail to reproduce.