Is there any chance that deleting the account I do not want will compromise/damage my main account ?

I don't think so.

Prior to deleting the erroneous account backup your entire Thunderbird profile, which you should do on a regular basis anyway. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Thank you very much. However I am not that 'techie' and do not understand how I can locate my profile and copy it if Thunderbird is shut down, as the information instructs me to do ?

See https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile It's the very same article. All you need to do is scroll up a little.

Hi again,

I have located my Thunderbird profile default file and I tried copying it to a disc, however, every time I just received a message 'Invalid MS-DOS function ?

Thanks for your help

You have to burn it to a disc, not copy.

Hi David,

Thank you. Can I drag and drop the profile file onto a disc, and if I do that will it remove it from the thunderbird folder or simply copy it across to the disc ?

Thanks again

Christ1's suggestion was primarily to ensure you caused no harm in your account removal. You can achieve that by simply copying to another folder on your PC. When you place a blank disc in PC, a window will likely pop up, offering a prompt on how you want to treat the disc.

If interested, here is more info on writing to a disc. https://www.howtogeek.com/689705/how-to-burn-a-cd-or-dvd-on-windows-10/

Done it, copied the profile to another folder, deleted the erroneous account in TB and no probs' Thank you !

I'm glad all is okay. And copying the profile periodically is the best way to recover from disasters, which always come unannounced.