404 Error on valid post URL's

Yesterday, I scheduled a post to be published after 1am this morning. It did not publish, so I manually published it (https://instantpracticebuilder.com/blog/update-on-the-rainmaker-marketing-protege-course-as-of-march-9-2014/). However, when I attempt to view the post, I get a 404 error. I created a second post (https://instantpracticebuilder.com/blog/update-on-the-rainmaker-marketing-protege-course-as-of-march-9-2014-2/) and published it immediately. Same thing, cannot view the post.

While editing the post, I can use the preview to view the layout, etc., and the post appears properly. When I visit the bog page, and hover over the title link, I see the correct URL, but when I click the link, I get the 404 error.

I get the same error whether I am using a fully qualified permalink, or the shortlink (https://instantpracticebuilder.com/?p=6927 and https://instantpracticebuilder.com/?p=6955). This also happens for the previous post, of February 7 (https://instantpracticebuilder.com/blog/why-you-must-plan-and-coordinate-your-strategy-business-model-and-tactics/), but not for posts of February 6 (https://instantpracticebuilder.com/blog/the-instant-practice-builder-metamorphosis/) and prior.

I cannot determine what happened between February 6 and February 7, but am wondering if it may have been a plugin test or update. Is there a record anywhere of post updates that shows the chronological point in which an update occurred. Looking at the last modification date on the server is no help as it appears many plugin folders are modified when run.

Any other ideas?

Update … before I even finish writing. This seems to be exclusive to Firefox. The links work properly in IE and Chrome

So, restatement of the question. Sny ideas why this would happen in Firefox? WP or FF, and where?

Many thanbks!