High CPU Usage when Tab Tree is open and idle, goes down when hidden

[gnl]

Reliably reproducible on my machine (Firefox 52.0.2 (64-bit), MacBook Pro Late 2015, OS X 10.11 El Capitan). CPU usage for Firefox goes up to constant 20-25% when the Tab Tree sidebar is open, goes down to low single digits/almost 0 when it's hidden or disabled.

[philippelatulippe]

I have the same issue, Firefox goes up to 25%-60% when Tab Tree is active, and back to 0-2% when I disable it. I have about a 100 tabs open in one window, and 4 in another window. The problem persists even after restarting firefox to make all tabs inactive.

I checked about:performance, but it completely underreports the performance impact, it thinks it's consuming about 2% when it's consuming at least 30%.