It stopped working for me too with FF 63. Then I noticed I was running some old version (probably auto update didn't work). Uninstalled, reinstalled to get the latest version and it's working again with FF 63.
TO USER: Best of I used!! already: custom key to hold TBC : delay in opening! delay in opening! delay in opening! waiting so long~~~ source: [Feature Request: Optional Delay For Opening Each New Tab · Issue #114 · cpriest/SnapLinksPlus](https://github.com/cpriest/SnapLinksPlus/issues/114)