I think I've found a memory leak in plasmashell when Firefox is used to download plenty of individual files (~30) and has the plasma add-on installed and enabled, which causes every download complete event to show up as a plasma notification.

I have recorded to videos showing the memory usage of plasmashell in the following scenario:

• download a bunch of images from a web page via DownThemAll - this allows to quickly download many files, hence triggering many download notifications (only when the plasma add-on is enabled)
• move to next page and download some more
• rinse and repeat a few times
• close firefox
• monitor memory usage by plasmashell during the test and at the end

When the add-on is enabled, a lot of memory is retained. The video shows 4 or 500 MB, but previously it went as high as 7-800.
When the add-on is disabled, memory usage increase by plasmashell is only a few MBs (although it might be worth checking if this gets properly released eventually...)

Suspected memory leak (add-on enabled):
https://drive.google.com/file/d/1P-bbsj ... p=drivesdk

Add-on disabled:
https://drive.google.com/file/d/1KFMn_L ... p=drivesdk

Plasma version: 5.19.5
Frameworks version: 5.74.0
Qt version: 5.14.2
Kernel version: 5.8.0-48-generic
Kubuntu 20.10

You should take this to the bug tracker instead, the forum is the wrong place to report this: https://bugs.kde.org