I just downloaded and installed Thunderbird 45.4.0 for both my Windows 7 and MAC El Capitan computers. I've been using Thunderbird with Windows for years. I was running v3.1.2 from Aug. 2010 (or perhaps I downloaded an old version then. I had no problems running the old version. I setup a new Windows box in Nov. 2014 with v31.2.0.
Today is probably the first time I've installed Thunderbird on a MAC. I did not setup the MAC as yet. I'm hoping I can sync it to my Windows 7 computer. Not tonight as I'm going to bed.
On updating T-bird on the my Windows 7 computer I got the following message...
Why we need donations
You might already know that Thunderbird improvements are no longer paid for by Mozilla. Fortunately there is an active community keeping it running and developing it further. But to survive long term, the project needs funding.
Thunderbird is currently transitioning to an independent organization. Being independent, we can shape our own fate, but there is significant infrastructure that must be maintained to deliver the application to our tens of millions of users. For Thunderbird to survive and continue to evolve, we need your support and ask for your donation today. All the money donated will go directly to funding Thunderbird development and infrastructure.
Hope this helps,
Dave Hartman
Above I said Thunderbird has been spun off. Close but no cigar: it's in transition.