I've had a response from Adobe Client Care, and they've clarified something that was incorrect in my original post: what Firefox is proposing to block by default is "Third-Party Cookies"; this is a different setting with the "Content Blocking" configuration of Firefox (screenshot below).
This policy appears to have no impact at all on the functioning of Adobe DTM, Adobe Analytics, and Adobe Target (as far as maintaining "Visit" and "Visitor" linking of activity goes).
I've done some reading on this and some testing, and I'm 99% certain that v65 of Firefox scheduled for release on 29th Jan 2019 will block, by default, Adobe DTM (and, therefore, all Digital Analytics and Marketing tags that an organisation delivers using DTM).
My reasoning is below; I would be very grateful if someone is able to tell me that I my conclusions are incorrect.
In summary, my conclusions are:
Firefox v65 will block Adobe DTM by default (and all content delivered by it)
Firefox v65 will block calls to Adobe Analytics (and other Digital Analytics tools) by default
To circumvent blocking of Adobe DTM-delivered content, an organisation must host the DTM file(s) on their own web content server
To circumvent blocking Digital Analytics server calls to *.omtrdc.net (the Adobe Analytics data collection centres), an organisation must have a CNAME redirect (e.g., metrics.myorganisation.com --> mycompany.d3.omtrdc.net)
Adobe Target will not function under Firefox v65 by default
Content Blocking in Firefox
The screenshot below shows the default setting in the current release of Firefox.
The "All Detected Trackers | Only in private windows" setting will block all tracking and marketing in Private Browsing; e.g., Adobe DTM, Google Tag Manager, Adobe Analytics, Google Analytics, Adobe Target, Facebook
With the setting demonstrated above, there are no problems with Digital Analytics and Marketing tags (including Tag Management Solutions) in "normal" (i.e. non-Private) browsing.
NOTE: in the blog posts and reporting around the new "Enhanced Tracking Protection" features, the specific version number is not often mentioned - the phrase "early 2019" or "January 2019" is used; the only release that fits these descriptions is v65.
The change that is anticipated is highlighted in the screenshot below:
Enabling this setting immediately modifies how pages will load when instrumented with some examples of a Tag Management Solution (TMS), Digital Analytics tool, or Marketing tag.
just to add here as with recent changes in Firefox regarding the content blocking if you choose the custom option and block the trackers in all windows then Analytics call/image will not fire. by-default standard option is selected so in that case, tracking will be disabled in private window but if user choose the custom option as per below screenshot then DTM/Launch call will not fire.
My interpretation of the blog post(s) is that it will only block slow-performing trackers, and third-party cookies (cross-site tracking). You are absolutely correct that once tracker blocking is enabled by the user that it will directly affect most implementations, including those on DTM and Launch. Firefox enabling tracker blocking across the board is purely speculation based on my research. Due to how many websites that have some pretty crazy dependencies on trackers loading, I don't know if they'd enable that by default.
That doesn't take away from the fact that they might actually do it, though. Let's keep a sharp eye on Firefox's latest nightly build to ensure tracker blocking doesn't spring any surprises on us.