I have been successfully using Avira AntiVir Premium for about 6 months on my humble PC (500MHz, max’d out at 288MB RAM) with the WebGuard and MailGuard disabled. The Avira on-access scanner automatically ignores Mozilla Thunderbird mail files, where there is one file per mail folder, some of which are hundreds of MB in size. Malware attachments are detected when read by a viewer application. Thunderbird optionally supports having messages scanned with the on-access scanner upon download from a POP3 account. See Download each e-mail to a separate file before adding to Inbox - MozillaZine Knowledge Base
For the following reasons,I suggest Comodo’s on-access scanner automatically ignores Mozilla Thunderbird mail files:
(1) Greatly reduces CPU and disk loading.
(2) Makes CIS easier to install, which is especially important to novice users.
(3) Given that some mail files are small (ones containing few messages), Comodo’s option of excluding from scan based on file size does not achieve the goal.
(4) HeffeD informed me that Comodo’s option of excluding a file from scan does not prevent its scan (CPU and disk loading) – only detection alerts. See https://forums.comodo.com/anti_virus_help/does_cavs_have_these_features-t32534.0.html
(5) Excluding from scan the folder containing Thunderbird mail files gives malware a place to hide from detection.
(6) Excluding from scan the specific Thunderbird mail files (which have no extension) leaves vulnerable newly created mail files in the future (unless the user just happens to remember this issue forever).
(4) is a deal-breaker for me. I won’t install the AV component of CIS until (4) is fixed.