[desktop] Bugs logged by Desktop Release QA in the last 7 days

Hello, Here's the list of new issues found and filed by the Desktop Release QA team in the last 7 days. Additional details on the team's priorities last week, as well as the plans for the current week are available at: https://tinyurl.com/tuwwhftm. Bugs logged by Desktop Release QA in the last 7 d

Intent to prototype and ship: Support `document.execCommand` and related API in `` and ``

Summary: We support `document.execCommand` and related API only with `HTMLEditor`, but accidentally it may work in `` and `` if there is `HTMLEditor` instance in the document. I fix this to support them without `HTMLEditor` like the other browsers. Bug: https://bugzilla.mozilla.org/show_bug.c