Make sure you are not blocking content.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

One issue that seems more common lately is Firefox's Tracking Protection feature. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature: Tracking Protection {web link}

Yes, the problem is still there, also in safe mode. (I have only one (1) adds-on).

I could add two things to the description:

Firstly, than when I try and connect to Yahoo mail I get redirected to the login page (and am stuck there). Normally I get directly to my mail page.

Secondly when I try to leave the login page and reach other Yahoo pages I get the Firefox message: "Firefox has detected that the server is redirecting the request for this address in a way that will never complete." (Yahoo cookies are allowed)

I have in earlier Firefox versions experienced that Firefox has protested that it believes that the Yahoo page is not configured in a secure way and that I hence had to allow for an exception. Which I always did.

Could Firefox recently somehow has stopped accepting such an exception and simply refuses connection to Yahoo?

Let's try this; https://support.mozilla.org/kb/Clear+Recent+History#w_how-do-i-remove-a-single-website-from-my-history

Remove History For One Site

Open the History Manager <Control><Shift> H. In the search bar, enter the name of the site. Right-click on one of the listings and select Forget About This Site. This should remove all information, including any passwords / settings.

Tried it now, but since I always automatically delete my history after every session, the result is nil. I (or Firefox) have already forgotten that site, as well as all other sites.

I think that either Firefox or Yahoo must have changed something in the last few days. I actually first noticed this perhaps a week ago or so. It started with an intermittent problem to connect. But then I just had to wait a while and then it was back to normal. Once I had to tweak the address a little, but then it worked again. In the end nothing worked.

I have been thinking to contact Yahoo also but I fear they will be less forthcoming.

I just noticed that if I write the address de.yahoo.com in the address field, I not only can't connect to the Yahoo main page, the address is also changed in the address field to the somewhat mysterious:

https://guce.yahoo.com/consent?brandType=eu&gcrumb=Bi0IXLk&lang

This doesn't normally happen in Firefox och other browsers.

What kind of address guce.yahoo.com is I don't know.

I searched that address on Google (ahem) and you're not the only one seeing it:

https://www.google.com/search?q=%22guce.yahoo.com%22&ie=utf-8&oe=utf-8&client=firefox-b-1

I tried https://de.yahoo.com/ And I ended up at https://de.yahoo.com/

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

After some testing on different computers with different settings I have come to the following conclusion:

Since some days or weeks ago, Yahoo obviously demands that you allow all cookies, not only from Yahoo, to access their mail service. At least it seems to work if you allow all cookies.

I normally only allow cookies from a selected few (e.g. Yahoo). I however noticed than when I used computers without these restrictions Yahoo worked fine.

I also noticed, after I had deleted all Yahoo cookies, passwords, etc from Firefox on my ordinary computer that Yahoo actually asked me to allow cookies, even though Yahoo cookies were already allowed.

I will naturally complain to Yahoo about this.

I hope this can help anyone who has experienced the same kind of problem.

Problem solved

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.