I have seen the same. I have uMatrix and NoScript (details below).
In my case, using the uMatrix logger, it fills up with MANY requests per second.
Like this:
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20:12:32 behind-the-scene xhr https://sync-messages.invalid/moz-extension://375aab03-55fb-4ac1-a2e2-a2647114f356/?id=34899cc575.837e%2Cabout%3Ablank&url=about%3Ablank&top=true&suspend=true 3p
I noticed it because it 'started the fan' in my PC.
CPU usage, reported by Windows Task Manager, went up to over 60%.
All of this was "firefox.exe".
Normal "CPU Usage", with 8 "firefox.exe" running, on a Profile with
NoScript 11.0.4rc13 (and uMatrix) is less that 2%, often 0%.
There are only 4 sync-messages.invalid since I started this Profile, more than 10 min ago.
On trying to document, I am having difficulty in reproducing.
I wonder if it is only seen 'soon after the Profile is launched' for the
first time that day? However, I have now seen it again.
Perhaps it is a 'race condition' with uMatrix?
All my Profiles have NoScript and uMatrix.
STR
With a Firefox ESR Profile that has NoScript 11.0.5rc3 (and uMatrix 1.4.0)
Both NoScript and uMatrix have a 'very short whitelist'.
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1. Start Windows "Task Manager", "Processes" Tab, "Show processes from all users".
2. Launch Firefox (with Home Page set to about:blank).
3. Open the uMatrix logger.
4. In a new window, go to https://arstechnica.com/2019/10/
5. In uMatrix, Temp allow scripts, images and CSS from arstechnica.com
6. In uMatrix, Temp allow images and CSS from cdn.arstechnica.net
7. In NoScript Temp allow scripts from both ... arstechnica.com and ...arstechnica.net
8. In uMatrix, Temp allow sripts from cdn.arstechnica.net
Keep an eye on the uMatrix logger.
At 4. there are many
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... arstechnica.com -- other https://noscript-csp.invalid/__NoScript_Probe__/ ...
This is normal (and expected by me).
At 6. the images, from the CDN will appear (again expected)
and the uMatrix logger includes:
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... arstechnica.com image https://cdn.arstechnica.net/wp-content/uploads/2019/10/sms-150x150.jpg ...
This is expected (the uMatrix logger is showing "image" from the CDN is now
alowed; no "--" in the log [for showing a block]).
I am still using NoScript to block scripts at this point.
At 8. the sync-messages.invalid starts to spam the uMatrix logger
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... https://sync-messages.invalid/moz-extension ...
I put more detail in the FIRST example at the top of the post.
Using about:support to document:
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Name Firefox
Version 68.2.0esr
Build ID 20191016163237
User Agent Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0
NoScript 11.0.5rc3
uMatrix 1.4.0
DJ-Leith
Edited to add more comments about uMatrix logger (at 6.).