I recently installed the latest version of JCE (2.6.33) on top of Joomla 3.9.0 and discovered that the guidelines you get around table cells when creating K2 and Acymailing articles to show their outlines were gone. I'm referring to the "Visual Guidelines" as described at:
https://www.joomlacontenteditor.net/support/documentation/editor/editor-content
I then just happened to look on a different computer and they were still present. Both computers are Windows 7, both are running Firefox 63.0. The one with guidelines missing is a laptop, the other a desktop. I tried clicking the "Toggle guidelines/invisible elements" button and it did nothing.
I couldn't figure out a solution so I decided to revert to my old JCE version, 2.6.30, and that fixed the problem on my laptop but then CAUSED the problem on my desktop.
I've been reading forums, googling, trying everything I can imagine and just cannot figure out what might be causing this. Has anyone come across anything like this and, if so, how do you fix it?
Try installing the Web Developer Toolbar and delete some cookies. Seems like it might just be a change in how that setting is stored: https://chrispederick.com/work/web-developer