User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Firefox/52.0 SeaMonkey/2.49.4 Build ID: 20180711183816 Steps to reproduce: Probably this is caused by a large number of tags (they were gathered automatically from some RSS feeds). The tab is remembered and opened by default every time the preferences dialog is opened. If I restart TB in "Add-ons disabled" mode, the issue is gone, but only until I visit that tab again. Actual results: When the tab is opened, the dialog hangs for a while, then stucks here. I can switch second-level tabs, either by clicking or by Ctrl+Tab, but clicking on top-level tabs does nothing, as well as clicking on controls in other second-level tabs (Formatting, Advanced). If applicable, there are messages falling to console: Error: A long running script was terminated menulist.xml:136:1 Error: Script terminated by timeout at: get_editable@chrome://global/content/bindings/menulist.xml:136:1 _selectPane@chrome://messenger/content/preferences/preferences.xml:826:19 observe@chrome://messenger/content/preferences/preferences.xml:756:17 menulist.xml:136:1 NS_ERROR_NOT_AVAILABLE: Cannot call openModalWindow on a hidden window nsPrompter.js:340 TypeError: Argument 1 of Window.getComputedStyle is not an object.[Learn More] preferences.xml:1296:39 Expected results: The tab doesn't prevent going to other top-level tabs.