You've hit on the relevant distinction: user context vs system context.

I use containers to separate system context (partitioning cookies and site data) while remaining in the same user context (e.g. "personal browsing").

I use profiles for when I need different user contexts (different bookmarks and frequently used sites for different clients or projects).

When only one tool is available, you are limited by the constraints of that tool (e.g. bookmarks bleeding over between user contexts when using containers, or having to copy extensions or bookmarks into every profile).

I like to have it both ways. Given the options available so far I've preferred to have bookmarks shared between both and mainly keep tabs / working context separate.

The best I've managed so far is to run separate instances of Firefox with the same Sync Account which appear as separate 'devices'. Nightly with a different icon and theme makes it obvious which is which.

This way if I come across something interesting I want dig into further on my own time, I can bookmark it as well as 'Send Tab To Device -> Home' or 'Phone' depending on where I want to be and what environment I want to have available when I see it later (e.g. read only, or hands on).

Yes, I may be hoarding a lot of tabs but that's a separate issue...