There should be a "grippy" control in the lower right corner to click-and-drag to resize the dialog.

It seems that the the dialog is sized and located to be centered on the page, so I wonder whether something in the page is causing Firefox to think is is wider than the first display. Is there a horizontal scroll bar at the bottom of the page (when no dialog is displayed)?

By the way, the column widths are stored in a file named xulstore.json in your profile folder, so if you run out of other options, you could reset various measurements by removing that file. Here's how:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > Help > More Troubleshooting Information
• (menu bar) Help > More Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in Windows File Explorer.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > Exit
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup, then:

Right-click xulstore.json then Rename to something like xulstoreOLD.json

Start Firefox back up again. The next time you show the Certificate Manager dialog, the column widths should be recalculated.

Restoring Toolbars

To change the display of the Bookmarks Toolbar and/or Menu Bar, either:

• right-click (on Mac Ctrl+click) a blank spot on the tab bar (or the "+" button)
• (menu bar) View > Toolbars
  On Windows, tap the Alt key or F10 key to activate the menu bar temporarily.
• in Customize mode > Toolbars button (see: Customize Firefox controls, buttons and toolbars)

Deleting/renaming the xulstore.json file did it - thanks. Of course it would be great if this wouldn't have happened in the first place, but glad to have this issue resolved.

This can happen if you dragged the page when the Firefox window was wider, if you the reduce the width then its width might not a readjusted. Note that you can try full screen mode or zoom out the page, I sometimes do that for seeing the right panel in the Security Device Manager.