I have a setup with three monitors, and some time a few months ago I believe after a firefox update I started noticing a problem with it specifically (I haven't noticed this with any other programs that generate multiple windows).

Sometimes windows that exist on secondary monitors will temporarily appear in the primary task bar. It can be triggered via alt-tabbing, otherwise focussing a window, or dragging a window from one monitor to another.

It doesn't happen all the time, it's not always all windows, and when they do appear they will fix themselves almost immediately but sometimes they won't disappear. While this is going on they don't ever disappear from the correct secondary taskbar where the window actually is so in effect I have two entries - one in the main, one in the secondary.

I've been able to reproduce this on a blank firefox profile so I don't think it's anything related to my profile. My monitor setup is 3x 1080p, middle on one GPU and the outer two on a secondary GPU. I also use focus-follows-mouse mode (aka sloppy focus), I haven't tested if that's related. I'm on Windows 7 SP1.

I'm not sure what diagnostic information would be useful, please let me know any information I can share that would help.

I don't think that firefox is hanging, at least if I have a video playing or something like that there's no interruption, but with firefox's multi-process stuff maybe one particular process is hanging.

I've attached the debug log, it's only about 15-20 seconds long and I was able to reproduce the issue multiple times in that period (a couple of times only a very brief flicker, once or twice the extra windows stayed for several seconds).

That option doesn't seem to change the behaviour.

Just to clarify, the windows never get removed from the DF taskbar except when I'd expect them to (when they move to the primary monitor). They only show up as extras on the windows task bar temporarily before disappearing again.

I recorded a video cropped just to the taskbars showing what I see. This has the taskbars stacked vertically top-to-bottom but that's just in the video to make it a less horrible dimension of 1920x150 rather than 5760x30, in my normal setup these are horizontal left-to-right in the same order.

I've also noticed now that I've recorded it (though you can't see in the video without seeing my whole desktop) that there's sometimes a delay between when a window is activated and it's shown as active on the DF taskbar. This is super minor since I hadn't noticed before, and I don't think it's necessarily related as e.g. if I click on the DF taskbar entries instead of alt-tab or clicking the window they are shown as active immediately but this bug still reproduces.

I also tried disabling focus-follows-mouse in windows, but it still reproduces there.