Does not change the time format in the email tooltip (with the "from:", "to:", "date:", "subject:" and "mailed-by:" fields).

Perfect! Did exactly what I needed it to do!

Way better than the default GMail setting, but it would be great if this add-on allowed one to choose the date format.
GMail is extremely limited: if one wants to have dates formatted according to ISO-8601 (YYYY-MM-DD) one has to change language to French Canadian or Lithuanian (besides some oriental languages).
ISO-8601 is the format prevalent in scientific work, and one quickly gets used to it, since it follows the usual little-endian number format which is universally adopted.
Chrome users have better luck:
https://chrome.google.com/webstore/detail/gmail-date-time-formatter/nkmaeogddfijadboccpafnmhkjhidgeb

To illustrate it better, a joke:
If the same logic used for dates was adopted for numbers, "three hundred forty five" would be
- 345 in ISO-8601 way,
- 543 in most others way and
- 435 in US way.

This is one of those extensions that is so valuable I forget about it. I recently switched from another browser and couldn't figure out what was wrong with my Gmail until I remembered this.

Very useful thing

Typing in Gmail had slowed to a crawl. After trying many things (emptying cache, restarting Firefox, signing out and back in to Gmail, etc.), I tried disabling Gmail Show Time. My problem instantly went away. I'm not saying that this proves a problem in the Add-on, but it sure deserves some attention. And if you find that Gmail suddenly slows down, disabling this Add-on is the first thing I'd advise doing.

Extremely useful, but slows latest GMail down with message "A web page is slowing down your browser, what do you want to do". Same if cookies and cache cleared.

Does exactly what I wanted. Thanks!