Thunderbird began improving, as far as I know, after multiple post-Mozilla iterations, including returning to the Mozilla fold in a different relationship, which is where it is now. Its improvement was not correlated with the Mozilla breakup.
Mozilla and the Thunderbird teams knew, however, that a breakup was needed for both Thunderbird and for Firefox (Thunderbird compatibility requirements were holding back Firefox).
The reason was that Thunderbird was a low priority for an organization that has a much more successful application in Firefox. Low priority projects don't do well; the organization naturally structures, invests in equipment, hires, manages, etc. for the high priority. You want to be a high priority wherever you are - better to be the big app in a small org than a small app in a big org (as a very general statement).
If you don't know these things, why make up sh-t about Mozilla? What do you get out of it?
Mozilla and the Thunderbird teams knew, however, that a breakup was needed for both Thunderbird and for Firefox (Thunderbird compatibility requirements were holding back Firefox).
The reason was that Thunderbird was a low priority for an organization that has a much more successful application in Firefox. Low priority projects don't do well; the organization naturally structures, invests in equipment, hires, manages, etc. for the high priority. You want to be a high priority wherever you are - better to be the big app in a small org than a small app in a big org (as a very general statement).
If you don't know these things, why make up sh-t about Mozilla? What do you get out of it?