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Firefox Got Faster for Real Users in 2023
(hacks.mozilla.org)
This is a most excellent place for technology news and articles.
Is it just bookmarks, or is it bookmarks and sync? I'd export your bookmarks as JSON or something and look through it, either with a text editor or some tool, and make sure there isn't some enormously huge string, or one containing unsupported characters, that's tripping something up somehow.
To be honest at this point I forgot. It's been a while since I dealt with this issue. I'll live with it as is.