Both of these point to some kind of malware infection of your system. Specifically one that has also likely compromised your ability to view secure/encrypted sites safely, which is why you're getting that warning message. Though I have run into the error you describe when trying to use Firefox at work where they have a proxy that basically intercepts even SSL traffic for virus scanning. Chrome on Windows just uses Internet Explorer's settings and security certificates, whereas Firefox has a complete top to bottom implementation of that. So in that sense, Firefox can be the canary in the coal mine, so to speak, by alerting you to an attempted man-in-the-middle type attack that both IE and Chrome will never trouble you about and allow your passwords and other sensitive content sent over seemingly secure connections to be siphoned off to who knows where for who knows what.
Still, just to be sure on the second issue, did you try manually navigating to the page of your yahoo bookmarks? Instead of using your bookmark, going to the main yahoo website and then making your way from there to whatever section of it you're looking for. It's possible that somehow along the line your Firefox bookmark was corrupted and simply needs you to recreate it.
I have no idea as to why the following strangest problems have started in my Firefox browser after I (successfully) reinstalled it.
1) The window with the message "This connection is untrusted" has started appearing (which then requires a few annoying steps to "add exception") when I access SOME web sites (like Yahoo!). Until a couple of days ago, that annoying warning window never popped up for any web sites, which was perfectly fine with me; for before accessing any web site I can judge for myself without the above "warning" message, about the trustworthiness of a web site. I had no problem in the past without that "warning." How do I get rid of it?
2) After the re-installation of Firefox, most web sites continue to appear properly on my screen, but when I access a certain bookmarked Yahoo web page, in particular, instead of it appearing properly as in the past, in the strangest manner, Yahoo's ("main"?) page containing a huge number of its links has started appearing, and further even far more annoyingly, when I click on a link in it, for SOME links, the page that appears contains an OVERLAY of the previous "main" web page so that the contents of the two web pages are garbled on the screen! I had never encountered the weird overlay problem in the past. My same bookmarked Yahoo web page does continue to appear properly on my screen when I use either Internet Explorer of Chrome browsers. After encountering the above problems with Yahoo in Firefox, I even changed my password for Yahoo, thinking of the possibility that somebody might have fooled around with my access to Yahoo in Firefox browser. Changing the password did not remove the stated problems. Thinking, next of opening a new account at Yahoo as a possible remedy (and then closing the existing account), when I tried to do so, Yahoo's registration would not accept use of any password from me within Yahoo's restrictions on the alphameric composition of a password!
I do not know if any experts can figure out, through imagination, the source(s) of the above multiple weirdest problems and then suggest solutions.