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An essential tool for multiple accounts Rated 5 out of 5 stars
BorderColors is a very light extension that adds a configurable border to any compose window, colour-coded to remind you what will appear in the From: field.
Configuration is trivial, one just has to assign colours to each email account; after this, the extension is failry unobtrusive but effective: the thin border is a bright alert, in case you are using the wrong account.
Extension has been working fine for months now, I'm surprised is still in the sandbox.