Here's how it looks like

Just noticed that Behance.net has this problem too

Oh, I was wrong, Twitter has this problem too(( Can somebody help me with this?? I'm using the last Firefox Developer, starting it in safe mode don't help too.

For Firefox 48+ see the next post instead

You're running Developer Edition? What I'm going to suggest toggling is an old Firefox 33 preference related to OMTC that may have been superseded in your version, but you could try it, especially if you have Nvidia graphics.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste layers and pause while the list is filtered

(3) Double-click the layers.offmainthreadcomposition.enabled preference to switch it from true to false

I can't recall whether that takes effect immediately or only after you exit Firefox and start it up again (since I don't have the symptoms to test with).

Sorry, those steps were old. Here's the update:

(0) Select and copy the following new preference name:

layers.offmainthreadcomposition.force-disabled

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste layers.off and pause while the list is filtered

(3) Right-click a blank area of the page, click New, then Boolean

(4) Paste the preference name, then click OK

(5) Select true, then click OK -- the preference should appear in the list, bolded, and user set to true -- if you need to remove this later, right-click > Reset it

I think you probably need to restart Firefox before that takes effect.

Thank you for your reply, but unfortunately, it didn't help(

But what did help, is enabling multiprocessing... nice turn Mozilla)