[Affected versions]: - Firefox: 61.0, 62.0b3, 63.0a1 (2018-06-27) [Affected platforms]: - Win 10x64, Ubuntu 16.04LTS, macOS 10.13.4 [Steps to reproduce]: 1. Launch Firefox; 2. Open the devtools-inspector (CTRL+SHIFT+C / Tools -> Web Developer -> Inspector); 3. Refresh the page; 4. Spam-Click on the Toggle the 3-pane inspector button. [Expected result]: - the devTools pannels are rendered and displayed properly [Actual results]: - the rules tab is multiplied and extras are displayed as empty; - the whole 3-pane section is empty; [Regression range]: - doesn't seem to be a regression; - a mozzreegression run gave the following result: https://bugzilla.mozilla.org/show_bug.cgi?id=1369945 [Additional notes]: - attached screencast with the issue; - step 3 can be skipped in some casses, or for step 4 to work a web-page with rich content needs to be accessed; - closing and reopening the devTools "resets" the issue;