Actually based on Chromium 69 - 360EEv12 is the one based on Chromium 78...
Basilisk 55/Moebius was actually born by MCP as a fork of a snapshot of Mozilla Firefox 53.0a1 (yes, the Nightly channel); there's very little, if anything, backported from Firefox stable 54.0 or 55.0; in fact, MCP, right after forking, started removing features, so that Basilisk 55 was even inferior to release channel Firefox 53.0, feature-wise (especially in what concerns e10s and WebExtension APIs) ... MCP just gave it an appVersion=55.0.x for purely "sensationalism" reasons ...
Today's Serpent 55, as maintained by Roytam1 (updated roughly once a month), is a mixed-bag, an experimental app where patches from Mozilla, TenFourFox and, mostly, UXP are being applied...
As for the matter at hand, Chromium-derived browsers are notorious for gobbling up RAM at the speed of light... By design, each tab is run in its own browser process, add to that several other processes needed by the browser core and extensions, add the amount of RAM devoured by your ad-blocker extension alone and you get the picture... Session restore in Chromium browsers is also an issue, because the application has to spawn, after initial launch, ALL these additional processes to "resurrect" the tabs present in the previous session...
I was first introduced to PCs when Windows XP was the OS du jour, with its IE6 fine (!) browser which, if you care to remember, did not support tabs - opening 150-300 browser windows was unthinkable at the time...
When "tabbed" browsers came into being, people started abusing the feature, many ending up doing, what was later called, tab-hoarding ; but websites of yesteryear were mainly static HTML pages, so the impact on consumed RAM (my initial XP box came with 512MB!) was small; this allowed tab-hoarders to continue the type of workflow they had been accustomed to...
But lately, certainly within the last 5 years, web-sites have turned into web applications in their own right, laden with omnipresent rich media content and heavy scripts; embedded videos and high resolution images are now obligatory to attain high Google-analytics figures (which is what drives webpage creation now), plus wizards to social media are everywhere (even here on MSFN...).
Browsers are being served huge blobs of (minified) Javascript, Web Assembly (wasm) code, huge CSS files, HTML5 video etc., that have to be decoded and rendered locally by the browser engine ! Especially the design of the (very popular) Social Media sites (Facebook, VK, instagram, twitter, tiktok, youtube e.a.), targeting mobile devices with touchscreens, with their "infinite-scroll" pages (where more content is loaded in memory as you scroll further), all that is a veritable menace to under-resourced and on "hardware-of-the-past" desktop machines...
I am mentioning the above just to highlight the fact tab-hoarding has become a lot more difficult these days, especially on older (32-bit) OSes (which, by their nature, have worse RAM-handling/allocating capabilities than the recent 64-bit ones...).
Don't get me wrong, I'm not passing judgement on @Cixert's workflow habits, just pointing out that his practice will simply get worse in the future, irrespective of browser/OS bugs...
Pre-quantum Mozilla forks, like the ones issued by Roytam1, are basically single-process applications, which, by that fact alone, makes them more lenient on system resources; if you only have a few vital extensions, an intelligently configured content-blocker, you try to stay clear of desktop-hostile places like the main Social sites, then I suspect you can afford to open more tabs there... But remember, single process means that if one tab crashes, it takes the whole browser with it!
On the subject of RAM consumption, let me also recommend the Lull-the-tabs legacy extension (by JustOff), which minimises significantly RAM consumption at browser start-up, especially when it tries to restore a large previous session...
Looking for a similar add-on for Chromium browsers, The Great Suspender Original
https://chrome.google.com/webstore/detail/the-great-suspender-origi/ahmkjjgdligadogjedmnogbpbcpofeeo
sounds promising, but I've not tried it myself, since I never have more than 25 tabs open in 360EEv12/13 (several of which are system tabs that don't consume RAM). Some more extensions to try are mentioned in:
https://www.makeuseof.com/tag/google-chrome-ram-memory-usage/
By the looks of it, Cixert has hit some session-restore bug on 360EEv11 that manifests itself under his specific usecase and/or OS configuration... In the following URL,
https://www.webnots.com/8-ways-to-reduce-memory-and-cpu-usage-of-google-chrome/
I urge you to read chapters 2+3+4 ; if the session restore bug kicks-in at 151 opened tabs, then make sure you close the additional ones before exiting the browser; as others have said, you have to adapt to the browser's capabilities, should you wish to continue using 360EE...
I'm not quite sure what is the exact OS/architecture this happens on, but later 360EE versions may fare better with regard to RAM management and session restore; have you tried 360EEv12 and/or 360EEv13?
At the end of the day, if you find that none of the 360EE versions quench your work-related absolute need for 300 open tabs, you should consider staying with/switching to a browser that lets you do it... We can only help up to a point here, sadly...
Best regards, stay safe