Why toggling deprecated prefs may fix other broken features

[phkhgh]

This is more of a "hmmm" question - or observation that I'm sure other advanced users have noticed a time or 2.
I recently posted about using a:visited {color: value !important; } in userContent.css to more consistently change visited links' color.

Later that day, it started having problems. It mostly stopped changing visited links - only changing visited links' colors on a few sites.
Any ideas why toggling a deprecated pref in about:config would make a feature work, when all relevant userContent or userChrome.css rules and or UI options were correct, but the feature or functions just wouldn't work?

I've seen cases after Fx updates of still valid (unchanged) styling rules in userChrome didn't work. Several times, after lots of head scratching, I copied the entire contents to a new userChrome file & it worked again.
Showing nothing was wrong or deprecated with the properties, values or syntax. Sometimes, copying the entire, unchanged profile to a new profile fixed odd issues after an update.

This time I found the fix? was toggling value of about:config pref, which I've seen happen several times.
The odd part here was that the pref I toggled was deprecated a long time - sitting in about:config, but never caused a visited link problem.

I was browsing about:config for any changes. I remembered "browser.history_expire_days_min" used to sometimes affect visited links' color, depending on the # of days entered.
Before just deleting the pref, I changed the value, restarted - but no fix. I increased the value by one, restarted, then back to the original value - restarted & this time every site tested used the color in userContent.css.

Before the multiple toggling & restarting, sites either used their own visited links color, or didn't change at all.
I've seen toggling active prefs to another value - then back, make features start working several times. But I don't remember toggling a deprecated pref - eventually back to the original value - ever affecting anything.

[dickvl]

There is no code for browser.history_expire_days_min in current Firefox releases.
https://searchfox.org/mozilla-release/s ... gexp=false

If fiddling with this pref worked then there are likely other issues with files in the profile folder.
If you still have prefs from that long ago then consider to start with a new profile and remove all remnants from older versions.