To the best of my knowledge your the second person in the last 5 years to make such a statement.

Where did you download from? http//:getthunderbird.com ?

Not impossible as any site may be hacked, but vanishingly small probability that the adware or malware was from Mozila Thunderbird. Much more likely you used an unofficial or scam site, or you already had malware on your computer.

See

• Installing Thunderbird on Windows
  (or Installing Thunderbird on Mac)
  • download from get thunderbird.com edit getthundebird.com
• You probably now need to consider
  • Remove a toolbar that has taken over your Firefox search or home page
  • Troubleshoot Firefox issues caused by malware

Just because you see something questionable in Thunderbird (and Firefox) downloaded from mozilla.org does not automatically mean it must have came with download.

It was likely either already on system, due to a extension install or update or a external software install or update.

If you download Thunderbird from say cnet aka download dot com then it is a known adware site in which they use their own installer to bundle crap with otherwise clean software.

If you used a search engine site to find Thunderbird then search engines like Google and Bing often have Ad links above the real search results and those Ad links can often lead to questionable sites.