hello, i don't think that you should have to worry, as a change to compress the file size of bookmark backups landed in firefox 32: Bug 818587 - Compress bookmark backups

Thank you so much. philipp, for giving all-clear.

As for the presumed bug in version 32, however, I did not manage to verify it. The link provided in your reply leads to a case fixed and closed in May. Did I miss a continuation page?

oh, sorry for the confusion caused here by using mozilla's lingo. all development in mozilla is handled through bugzilla, so a bug can either be a real problem that needs to be fixed or a new feature that is going to be implemented.
in this case "bug" 818587 just documented the work of shipping the feature of compressed bookmark backups & and it was "fixed" in may, because this was the time when it landed in the early nightly builds (those are a bit like alpha versions of the software) for firefox 32...

Note that compressed JSON backup have a .jsonlz4 file extension to distinguish them from uncompressed backups that have the .json file extension.

You can still create a normal uncompressed JSON backup manually in the Bookmarks Manager (Library) as the compressions is only used for automated backups.

Ok, bookmark backups are compressed only if automatic. This explains why the manual backups that I hurried to create, still attained the former file size level.

Good to learn about the recently introduced .jsonlz4 file extension. I would not have been alarmed if the extension had been visible at first sight. However, file names got so long that only their left part is displayed in a column on a display with a low resolution.

I'd expect other users will fall into the trap the same way as I did. So hopefully their google search will deliver this thread among the first 20.

Thanks to all posters for deepening my understanding of the fox.