I honestly don't know what you mean by classical mode. I have been using Enigmail for years and only had it enabled on one account for one identity and I don't recall any setting to the otherwise. That said, I'm reporting my experience after merely upgrading to TB v60.x.x - a decision that was effectively made for my by Mozilla not knowing what I was in for. Upon restart I discovered many incompatible addons and the requirement to update the ones that remained compatbible. Enigmail was updated to...
Images disappear when saving encrypted drafts
Enigmail enabled by default for all email accounts/identities
pinenetry.exe can't start because libassuan-0.dll is missing
OK, as it turns out, Gpgp4win v3.1.5 is defective - my bad. I installed v3.1.4 and it now works.
pinenetry.exe can't start because libassuan-0.dll is missing