w3schools.com reports numbers based on its log files (i'd say the visitors of this site represent a specialized subsegment of total Internet community, from which it's hard to make a generalization about an average Internet user).
I tend to think that numbers reported by netmarketshare.com (which show that in 2018 Windows 7 has still been the most widely used OS) are more reliable:
https://netmarketshare.com/operating-system-market-share.aspx?options=%7B%22filter%22%3A%7B%22%24and%22%3A%5B%7B%22deviceType%22%3A%7B%22%24in%22%3A%5B%22Desktop%2Flaptop%22%5D%7D%7D%5D%7D%2C%22dateLabel%22%3A%22Trend%22%2C%22attributes%22%3A%22share%22%2C%22group%22%3A%22platformVersion%22%2C%22sort%22%3A%7B%22share%22%3A-1%7D%2C%22id%22%3A%22platformsDesktopVersions%22%2C%22dateInterval%22%3A%22Monthly%22%2C%22dateStart%22%3A%222018-01%22%2C%22dateEnd%22%3A%222018-12%22%2C%22segments%22%3A%22-1000%22%7D

I haven't been able to reproduce the issue you mentioned about favicon.ico. For example, i just visited a site and checked request headers sent, & everything seems functioning OK:
Host: static.bfmtv.com
User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:64.0) Gecko/20100101 Firefox/64.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: fr,fr-FR;q=0.8,en-US;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate, br
DNT: 1
Connection: keep-alive
Cookie: TCPID=119131139489067326342; xtvrn=$548539$; xtan=-; xtant=1; etuix=9cc_5.jv5oCi69wkDT9jtGVIJNVLip69oIcOR5ViDZO_4giNojVnug--; ownpage_fp2=7ebfab6931223c16
Pragma: no-cache
Cache-Control: no-cache
TE: Trailers

11/15/2017: I've been able to implement it as a WebExtension, but some minor nice-to-have traditional features couldn't be implemented so far. If the bug i mentioned below is fixed, it might be easier to re-implement the traditional features.

11/03/2017: I will try to convert it to a WebExtension, but as of now, WebExtensions cannot set preferences, which means there's no standard way to set navigator.* properties. You can vote for the following bug to request this feature for WebExtensions:
https://bugzilla.mozilla.org/show_bug.cgi?id=1414078
Your options will probably be:
1. If i convert to a WebExtension, you could use it with HTTP User-Agent header spoofed, but navigator properties not spoofed.
1.1.Until the bug above is fixed, I will try to provide a user.js file (probably somewhere under
https://github.com/haqer1
) so that users could spoof navigator properties as well by saving that file under their profile folder (with manual updates required until the bug above is fixed).
Once the bug above is fixed, i would try to release an update also spoofing navigator.* properties.
2. Switch to an ESR release, & continue to use pre-WebExtensions Blend In (but values will get more and more obsolete over time).
3. Not to spoof anything (i.e., not to try to blend in (perhaps until the bug above is fixed)).

This is also mentioned in About this Add-on section.

But i'll try to automate this for v41 (or v42).

Follow-up on ADD:

I just tried this using v40.0.3t38.2.0 on Linux, and it works as expected:
1. With thunderfoxing: "Firefox/40.0"; build-id is "20150826023504"; oscpu is "Windows NT 6.1; WOW64", etc.
2. Toggled thunderfoxing to false, quit, & started again: "Thunderbird/38.2.0"; build-id is "20150813074416"; oscpu is "Windows NT 6.1; WOW64", etc.
This stuff is shown under .applied, but better checked by setting Start Page to a URL that shows this stuff.
So i'm unable to reproduce this issue. Perhaps worth trying with a new profile (-ProfileManager)...
Also, will try to set up a place for bug reports (like there is for my IA addon) some time in the near future...


FYI, the just released v43.0t38.4.0 has the following 2 new features which you might find useful:
* Thunderfoxing setting's changes now take effect immediately (re-start is not required).
* Settings can now be viewed on a newly added local page (as opposed to having to view them by accessing a 3rd-party page): resource://blend-in/blended-in.html.

The URL from the 2nd feature, can be set in Tools->Options (Edit->Preferences): Location & accessed conveniently by using
Alt+Home
shortcut.


Thank you for your feedback.

Since buildIDs are version-specific, & latest agile release is the most common, the current implementation seems adequate for blending in as far as buildIDs go.

Users who'd like to stay at an older release or ESR release AND not blend in their versions with the crowd can disable auto-updates for Blend-in in Shift+Ctrl+A or globally by setting extensions.update.autoUpdateDefault;false in about:config.

Users who need to temporarily unblend in for compatibility or software download reasons, etc. can toggle
notApplicableIfRealPlatformIsWindows
or
applicableIfRealPlatformIsNotWindows
in about:config.

P.S. I've tried to keep this project low-key, but could consider registering it on mozdev.org, for instance, for enhancement requests, etc.

I haven't had any issues with Google Maps and Blend In. When you are having an issue, try toggling notApplicableIfRealPlatformIsWindows in about:config page; if you are still having an issue after that, then it's not related to Blend In.

P.S. If you can come up with the actual steps to reproduce, feel free to post them; but i'm quite certain it's not a Blendin' issue. :)