Mozilla, Thunderbird, and/or the EFF,
Do you know that donation to Mozilla don’t (and can’t legally) fund Firefox development, right? (opposite to what happens for Thunderbird, actually).
But, hey, you can guarantee Mozilla’s CEO a better retirement:
Thus, your money are better spent donating to Thunderbird or EFF or whatever other foundation that does something actually useful.
I mean… Why would you ditch a service you evidently NEED just because some stranger on the internet told you so?
That said, there’s plenty of offline programs that can work on .dock files, such as OnlyOffice and LIbreoffice (with varying degrees of compatibility with existing files). Good luck!