can I safely delete thunderbird from 20.04.3 LTS
Solution 1:
Yes, if you don't use it you can safely delete it. The only negative side-effect is that you will not have any application in the system to handle "mailto:" URLs, if you ever happen to click on one. A working mail client is required to handle these URLs. (But many people live without it, using only web-based mail as you are ;)).