The global "Default zoom" setting in Firefox gets cleared when running Cleaner with the "Firefox" "Site Preferences" option checked.
To replicate, first change the global setting in Firefox here: Tools > Settings > General > Zoom > Default zoom
Replicated in Firefox version 103
Well personally I would class the zoom setting as a 'Site Preference' so wouldn't be too surprised at that happening.
I'll have a play to see if I can replicate it and then if I can find a way that you can stop it happining.
Some interesting results from my testing.
I set the zoom to 150%, closed Firefox and then ran an analyze in CCleaner of just the Firefox Site Preferences.
The only file it listed under 'Site Preferences' for Firefox was "SiteSecurityServiceState.txt" - which is a 0kb file and on opening it it's empty (as you would expect with 0kb).
BUT clearing it with CCleaner did reset the zoom to 100%.
To double check I reset the zoom to 150%, closed Firefox, and manually deleted that file without using CCleaner.
Reopened Firefox and this time the zoom had remained at 150%.
Repeating and watching just what CCleaner is removing from that directory when it cleans 'Site Preferences' it is only removing that one file.
Very strange that deleting the file manually gives a different result than having CCleaner remove it?
I'll think of some more testing, but for now my best suggestion is to untick Firefox 'Site Preferences' if you want to keep the zoom level.
OK, after runing CCleaner in debug mode and looking at the log produced I think I can see what's happening.
It isn't removing that "SiteSecurityServiceState.txt" that's causing it at all.
As well as deleting that particular file, cleaning the 'Site Preferences' appears to be resetting the javascript file "prefs.js".
I can't think of any way to prevent that happening other than unticking 'Site Preferences' for Firefox.
I was able to do some more testing as well, and it seems to be the "content-prefs.sqlite" file that affects the setting (tested in Firefox version 103).
When running CCleaner with the "Firefox" "Site Preferences" option checked, it resets the "browser.content.full-zoom" property that's in the "content-prefs.sqlite" file. After, if I manually restore a backup that I previously made of the "content-prefs.sqlite" file, it restores the Firefox global "default zoom" custom setting that I had previously set.