Not an issue only a userContext Container oddity I cannot understand

[ghost]

Hi,

Environment: Temporary Containers 1.9.2 / Mozilla Firefox 115.4.0 ESR / Windows 7
Temporary Containers runs perfectly well, set here to Automatic Mode. An everyday pleasure.
I've installed the userChromeJS 'protocol' as proposed by xiaoxiaoflood - userChromeJS - Install & scripts @GitHub

This post only to expose an oddity which seems to be inherent to Firefox userContext Containers in general, but seems only (hereafter explained).
What I notice is that neither the Temporary Containers extension nor the Firefox Multi-Account Containers extension (which I don't use but have tested) are able to containerise a local file (file:///).

From there on, two oddities (in my understanding, I'm not techie):

1- ODDITY-1: previously to temporary Containers I had installed and used the xiaoxiaoflood - privateTab.uc.js userChromeJS userscript and was able to containerise a local file (file:///), so it seems technically feasible.

2- I use the xiaoxiaoflood - newtab-aboutconfig.uc.js userscript which allows the user to set Firefox's newtabe page. I've set accordingly:

// SET CUSTOM NEWTAB with 'newtab-aboutconfig.uc.js' (creates 'browser.newtab.url' preference in about:config)
pref("browser.newtab.url", "file:///F:/firefox/homeboard.html");

// SET CUSTOM HOMEPAGE = CUSTOM NEWTAB
pref("browser.startup.homepage", "file:///F:/firefox/homeboard.html");

Hence, Firefox's Homepage = Firefox's Newtab page and all runs flawlessly. But no way to containerise this local file.
ODDITY-2: if I open a whatever site, close the Homepage/Newtab page, and from the opened site hit the Keyboard shortcut Alt + Home ... the homepage is containerised!

Screenshot-1: Homepage/Newtab page, initially, not containerised:

Screenshot-2- Homepage/Newtab page when accessed with Alt + Home, containerised:

These two oddities truly puzzle me and I'd really be most grateful for an explanation :)