One possible cause is that the zoom level of the browser window exceeds 100%. Setting the zoom level of the browser window to 100% should resolve the issue.
Thanks for the suggestion, unfortunately zoom is at 100% and we're still getting the issue.
Do you have any extra add-ins installed?
I've left my Firefox locked at v49.0.2, seemed to be a bit glitchy after that, i've rolled back and switched off updates.
Nope, no additional add-ons. I'm pretty sure I can downgrade to resolve the issue, I'm just not sure how easy that will be for our users.
Go to about:config and change the layout.css.xul-box-display-values.content.enabled value to true. That did the trick for me.
Could you give us more details about to enabled this option?
I believe that the issue is related with the Firefox version greater than 60.4.
We can see the error in Firefox v64 but not Firefox 60.4.
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Latest update... the tabs reappear if you increase zoom to 120%...
Interesting about zooming in. That usually breaks more than it fixes.
I just got around to testing and I can confirm that the layout.css.xul-box-display-values.content.enabled setting does fix the issue.