The isn't exactly a tremor it's a refresh, bad visual effect, you can look at right side of window under tab buttons in tab bar. It's like a resize of page window. This happend also when closed tabs with middle click.
And the refresh icon (Loading) in tabs continue in background when opens a link in a background tab. The icon site is over refresh icon until you move the mouse over the tab, then tab button is refresh and refresh icon in background disappears.
I thought that this problem had already been solved by Dorian in 1.5 final.
K-Meleon in Spanish