Hmm, that shouldn't happen. A few questions:

• Is this on a particular site that might have special handling for right-clicks, or is it on all sites?
• Does right-click work normally other than on links, for example, on the background of the page or in an editing area?
• Could any of your extensions or user scripts be changing how right-click works?

• Single right-clicking on links in most places brings up the context menu (with "open URL in new window", etc.) for a split-second, then just opens the URL in a new window. Occasionally, it will leave the context menu open long enough for me to choose what to do, but most of the time, it just goes straight for "open in new window".

• Single right-clicking on other areas brings up the context menu with the back and forward arrows, automatically chooses the back arrow, and moves me back a page.

• I don't know how to check the third question fully, but most of my extensions are disabled, and I don't think any userscripts are running.

I disabled all my addons, and tried single right-click on "Quote" here. The context menu popped up for a split-second, and then opened it in a new window.

Hmm, if I think about stray keystrokes coming in, they're not consistent.

To activate the back arrow using the keyboard, you would press down arrow then Enter.

To activate Open Link in New Window requires two down arrow presses and then Enter.

What the heck is going on?

I don't suppose you've tried shutting down and restarting your system since this began?

And now... it pops up the context menu, and lets me choose which one. One of the addons may have been slow to disable itself?

Perhaps "Archive URL" Addon could be the culprit, making the menu big enough that sometimes the mouse is over the menu while releasing the button. Still testing.

Well... looks like it's got to be one of the addons. With all of them disabled, context menu appears to be working properly more often.

Still not quite perfect. Some times it still just opens the menu with the cursor over a menu item, and goes straight for that. At least it's now "most of the time it gives me a choice", so it's an improvement.

Easiest method to fix appears to be to toggle ui.context_menus.after_mouseup to true in about:config.

I think this will probably have the drawback of rightclick drag not allowing a choice of menu item, but should make the context menu auto-choice stop happening.

In fact, I'm fairly sure it did, as this time one right-click on "Quote" link here, gave me the context menu with a blue chooser line already over "Open link in new window", but did not automatically go do it. I'm terrible at right-click drag, so this is probably a fair solution.

GuardianElm said

Easiest method to fix appears to be to toggle ui.context_menus.after_mouseup to true in about:config.

Interesting. That setting seems related to a difference between Linux and Windows. On Windows, the context menu doesn't appear until after you release the mouse button (what you get by setting true). We can't select a context menu item using right-click-and-hold, move mouse pointer over a menu item, release. It's always a second click for us to activate a context menu item.