Thunderbird 60.3.2
Windows 10 latest.
QF Pro, paid. Version of 24 Nov 2018

I've used several of the icons recommended in QF.
In each case, the selected icon displays correctly in the folder tree.
Upon restarting Thunderbird, QF "knows" that an alternate icon does exist for
Folder A (for example), because the toolbar has the "remove customized icon"
icon.
But the displayed icon for Folder A has reverted to the default icon.

Troubleshooting thus far:
I've experimented with various settings for QF, including foldertree icon
settings.
I've placed the icons in various folders, and Setting shows that QF has the
correct path.
The correct path perists between Thunderbird sessions.
I've ruled out interactions with 
Dorando keyconfig,
Nostalgy,
Theme & font size changer.

I misunderstand the usage for this feature.
Ir functions well, but only for folders that are QF tabs.

Let me reopen that one. I think I am storing the necessary information fpr the
tree icons into the folder properties directly (so they are not stored in
QuickFolders settings). There may bee a problem with regenerating the css rule.
I will re-open and investigate.

In my recollection, it should be possible to "iconise" also folders that have
no QuickFolders Tab. As a workaround, yuou can start using QF Tab Categories
and create a "hidden" Tab by adding it to the "Hidden" Category. Then you can
still use icons and even advanced tab (folder) properties such as a customized
identity or a default "To:" address, without the need for displaying a tab on
the QF toolbar.

Assigning the bug to me. We really should be able to style as many icons in the
tree as we want, regardless of whether QF tabs are defined for them.

Thanks for the kindness!
Your statement that you store properties in the folder itself . . .
I see an entry in the local msf file (do you want it?). 
I actually have **two** IMAP clients on my computer. And therefore I keep
**two** local copies, one in Thunderbird (with the msf file) and a separate one
in Outlook.
In addition to the copy on the Outlook365 server.
Perhaps these two local copies cause the problem? 
Otherwise, the two-client arrangement works very well.

(In reply to comment #2)
> Let me reopen that one. I think I am storing the necessary information fpr the
> tree icons into the folder properties directly (so they are not stored in
> QuickFolders settings). There may bee a problem with regenerating the css rule.
> I will re-open and investigate.
> 
> In my recollection, it should be possible to "iconise" also folders that have
> no QuickFolders Tab. As a workaround, yuou can start using QF Tab Categories
> and create a "hidden" Tab by adding it to the "Hidden" Category. Then you can
> still use icons and even advanced tab (folder) properties such as a customized
> identity or a default "To:" address, without the need for displaying a tab on
> the QF toolbar.
> 

The "hidden" ploy is sub-optimal, because it would force me to reclassify each
of the many subfolders of the folder whose icon I originally wished to change. 

(In reply to comment #4)
> Thanks for the kindness!
> Your statement that you store properties in the folder itself . . .
> I see an entry in the local msf file (do you want it?). 
nope - Thunderbird doesn't store this information there. THe main thing is that
it get's persisted there and can also be read back from within Thunderbird;
where the information is stored physically doesn't really matter to me.

> I actually have **two** IMAP clients on my computer. And therefore I keep
> **two** local copies, one in Thunderbird (with the msf file) and a separate one
> in Outlook.
> In addition to the copy on the Outlook365 server.
> Perhaps these two local copies cause the problem? 

no I don't think so. This is an internal thing within Thunderbird, completely
independent from Account / Mail Provider storage

*** Bug 26622 has been marked as a duplicate of this bug. ***

*** Bug 26622 has been marked as a duplicate of this bug. ***

Created an attachment (id=8590) [details]
4.12.2 prerelease 30

The error may have been caused by outdated iterator Syntax.. this ve4ersioin
may fix the bug, please test.

Created an attachment (id=8599) [details]
4.13.2 prerelease 35

Found a problem with the storage that is caused when the icon is set from the
QuickFolder Tab context menu. It appears to be working now, finally.

note: I had to remove all Asian locales in this version because there is a
1000kB limit for attachments. This version will work in all locales except,
Chinese, Japanese and Vietnamese

It seems to work well for me too.
thanks a lot!

Resolve FIXED with Release 4.13.2 at 25/02/2019