See title. All custom icons that worked in Thunderbird 52.9.0 are not displayed
in the folder tree of Thunderbird 60.0b9

If the options menu command "Repair folders" is used the icons become visible
again but this is not persistent by restarting Thunderbird.

Also, looking at style editor in developer toolbox the file
qf-foldertree.css only lists the 2 hard coded existing rules and none of the
added rules. In the same on Tb 52.9 it counts 236 rules (but only shows the
default rules and also can't be saved for some reason).

maybe we can use the debug switch "folderTree.icons" to garner more information
on startup...

Created an attachment (id=8509) [details]
4.11 prerelease 89

This will start a timer to repair all tree icons (delayed by 10 seconds) in
Thunderbird 60 and greater.

Created an attachment (id=8511) [details]
1.6 prerelease 96


made the delay before refreshing the icons configurable. 

Open about:config and edit the value

extensions.quickfolders.treeIconsDelay

default is 7500 (ms) which works for me but shorter delays may be possible.

# The refresh is only in done in Thunderbird versions >= 60, as it isn't
necessary in Tb 52.*
# when refreshing too early, it can lead to the CSS of QUickFolders not working
(Tab buttons and toolbar buttons unstyled)

Fixed in 4.11, released on 16/10/2018