I wish they would not. I don't want any UI changes on my email client. Ever. It is one of my most important tools for work, so I don't want to tolerate anything that breaks or changes my workflow.
I am super happy user of thunderbird but if they keep doing this "modernizing" thing I will have to switch to a more stable mail client (after my current version that I never plan on updating breaks that is).
It is an application that receives content from the Internet, your advice is dreadful. Email is a primary vector for exploitation.
The modern example of fuckhead UI changes. Changing settings without having to click apply and/or OK. Changing from the "cog" icon to those three idiotic bars (hamburger?).
Interfaces are often used by people who are not IT savvy. The simplest of UI changes can mess their decade(s) old patterns of usage, the elderly are the easiest example of users affected by this. It not just their individual problem, those people then reach out for help which can be at considerable personal financial expense.
I dream of a future when normal interfaces and drop-down menus return.