Hi aboriqua:
I use Firefox as my default browser and posted about this cleaning of bookmark backups on 07-Jun-2022 post in Steve1209's CCleaner 6.00.9727 Keeps Reporting Firefox Cleaning?.
I would advise that you prevent CCleaner from cleaning your Firefox bookmark backups unless you are critically low on free disk space. If your Firefox bookmarks are ever corrupted or go missing you can restore them from a recent backup (Bookmarks | Manage Bookmarks | Import and Backup | Restore) as described in the Mozilla support article Restore Bookmarks From Backup or Move Them to Another Computer. As nukecad noted, Firefox only keeps ~ 15 of the most recent bookmark backups and regularly purges older backups, and my bookmark backups use less than 15 MB of disk space.
Firefox automatically backs up my bookmarks approx. every 3 or 4 days on my system and saves the .jsonlz4 file in my Firefox user profile in the hidden folder C:\Users\<<em>myusername</em>>\AppData\Roaming\Mozilla\Firefox\Profiles\<<em>firefoxuserprofilename</em>>\bookmarkbackups. I go one step further and manually backup my Firefox bookmarks (Bookmarks | Manage Bookmarks | Import and Backup | Backup) about once a week and save them to a removable USB thumb drive just in case some disaster happens and I am unable to restore the most recent bookmark backup stored in my Firefox profile (<em>note: if you are creating important bookmarks that you don't want to lose then a weekly manual backup may not be frequent enough for you</em>). If you are critically low on free disk space and allow Firefox to clean your bookmark backups just be sure you ways have a recent manual backup of your bookmarks safely stored somewhere like a removable USB thumb drive.
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64-bit Win 10 Pro v21H2 build 19044.2251 * Firefox v107.0.0 * Microsoft Defender v4.18.2210.6-1.1.19800.4 * Malwarebytes Premium v4.5.18.226-1.0.1823 * Macrium Reflect Free v8.0.7090 * CCleaner Free Portable v6.06.10144
Dell Inspiron 15 5584, Intel i5-8265U CPU, 8 GB RAM, 256 GB Toshiba KBG40ZNS256G NVMe SSD, Intel UHD Graphics 620</span>