I am not saying those bugs are not already bugzilla'd or even resofixed. I am just listing my top annoyances in existing versions of the products. For Firefox:

  1. when Firefox is launched for the first time from Thunderbird clicking on an hyperlink in an email, I often end up with two Firefox windows, one of them being blank and missing personal toolbar and other UI.
  2. when the password manager stores more than one account/password for a login page, the master pasword is prompted only after you type a first char in the account text field. Should be the contrary.
  3. sometimes, I don't even see that a given extension has options because those options are available through the Extensions dialog. I think this is an error. If Firefox has extensions with options, the Options menu item in Tools should be followed by a "Extensions Options"s entry.

Thunderbird

  1. by far the top one: the Order Received column does not display the sort arrow, and the sort order is often forgotten by the application. No, it's not always forgotten, only often. But it's very annoying.

  2. the CSS styles applied to an attached HTML document and viewed "inline" are also applied to the body of the mail... It's only one example of the bad side-effects: positioned elements are badly positioned (example: a confidentiality seal on such a document) and it can make the mail's body hardly readable.
  3. when sending mail fails because of a network problem, I often end up in a state where I must save the mail, close the window, and open the saved message as new to be able to do something with it. Otherwise, the toolbar buttons of the original message window are disabled or inactive, and it's impossible for instance to select another sender's email account and click on Send. Not deterministic.