Please provide a screenshot.

See this support article. How do I create a screenshot of my problem? It is best to use a compressed image type like PNG or JPG to save the screenshot and make sure that you do not exceed a maximum file size of 1 MB.

Then use the Browse .... button below the Post a Reply text box to upload the screenshot.

Still works for me in the current beta build (40.0b7) if I select multiple items in the right pane in the Bookmarks Manager (Library).

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thanks @cor-el. You were right. An extension is causing a problem. Until I have time to detect which one it is, I have rolled back to Firefox 38.0 which still works.

Thank you for your help.

You're welcome.

Does this extension or theme also cause this problem in the current 39.0 release?

You can check if there is an update, possibly on the versions page on the Add-ons website.