Any idea whether it is going back to the standard Firefox default setup or to something you did previously. Or and it would be very unusual, are all the icons disappearing and you do not even have the home icon and the bookmarks icon.

Are you saying that when Firefox opens initially your icons are as you expect but when you open a new Firefox window in that same session the icons disappear - so you can switch between Firefox windows or display them side by side and one will have icons one will not. I definitely do not understand if that is happening. I do not recall noticing anyone reporting that previously. Is this only on specific websites ?

I am not sure this article is directly relevant to your situation but see

• Changes to toolbars and window sizes are not saved

Here is a tip when customizing the toolbars, assuming you have the Search Bar on the Navigation Toolbar. If you have the Search Bar hidden in the Customize palette or Menu Panel, the following doesn't apply.

Don't put buttons/icons between the Location Bar and the Search Bar. Buttons in that area can mess up the "auto-resizer" feature for the Location Bar /Address Bar combination that permits the two of them automatically getting smaller when the browser window is made to be at less then Maximized width.

Using that area for buttons can also provoke the "overflow" on the right-side of that Toolbar (the "Navigation Toolbar") where you get a double-elypsis >> and buttons can drop off the end of the toolbar.

And sometimes using that area for buttons can provoke buttons to be lost the next time Firefox is opened or when the Firefox window is made narrower - Restore Down - and then back to Maximized.

This issue isn't "new" to Firefox 50, I have been experiencing what I described since Australis was introduced in Firefox 29 back in April 2014. I had to change the manner in which I had been doing Toolbar customizations for 12 years.