I’m running only latest avast 11.1.2245 so I don’t know if this applies to older versions as well, but the scan for outdated software reports that a new version of Thunderbird is available. However, the version that it says is “new” is 38.5.1, which is what my copy of About Thunderbird reports. This has been happening for about 3 days. Streaming updates are reported as recently as today so if that’s where the software release info gets updated, then streaming updates would seem to have it wrong. Or that info comes from somewhere else?
Expand the Thunderbird entry in the Software Updater to see what version it is saying that you have and what is available ?
I have Thunderbird 38.5.1 and it isn’t reporting that is out of date.
Try using the Refresh option at the bottom and see if that makes any difference.
I’ve found the Software updater to be a ‘little’ off at times.
I also have 38.5.1 as “up to date”.
For me it’s no real issue. 
@DavidR
Indeed, Software updater thinks I have 38.5.0 version. Refresh option had no effect. When the next version of Thunderbird gets installed I’ll check the expanded entry to see if its been updated. Thanks for reminding me about that feature.
I had what sounds like the same problem on 2 of my systems (Win 7, x64). Here it was a problem with the update to Thunderbird 38.5.1 from within Thunderbird, earlier this month. In my list of installed programs (Control Panel>uninstall a program) there were 2 versions of Thunderbird listed: 38.5.0 & 38.5.1.
The solution was to download the current version of Thunderbird from Mozilla and install it over the top of the existing installation. On the second machine I told Avast to update Thunderbird. Both approaches cleared the problem, removed the reference to 38.5.0, and quietened Avast.
You’re welcome.
@Michael529,
Thanks for sharing your solution to this issue. I’m also on Win7(two computers) and sure enough, Thunderbird had two entries in the Programs manager of Control Panel. I tried the same update methods that worked for you and, voila!, both the Avast (re-)install and Mozilla (re-)install took out the 38.5.0 entry and Software Updater is now happy. Me too!
Not to pursue this too far off topic, but one of the two computers has two active accounts and only one of them showed the extra Thunderbird entry. I have always assumed that “programs” were common to all the accounts on the computer so should I be looking for something nefarious to be lurking in the weeds of that system? or is this a one-off MS or Mozilla glitch? My routine Avast & Malwarebytes scans are always clean.
Thanks for your kind words; pleased to be able to help.
I don’t know what happened with the Thunderbird internal update but I have been assuming, on the balance of probabilities, that it was a Mozilla glitch. In some ways that thought is reinforced as we both had it and both our systems show clean. I don’t think that necessarily programs do appear in all accounts so that probably isn’t a significant finding on your system.