This started about 6 months ago, I would have to delete the browser history if I wanted to check my mail. it has gotten where I have to go right to my yahoo mail to check for emails. if I do anything else before checking my mail I have to either delete browser history or to shut down the browser (Firefox) and then reopen to be able to check my mail. I have found that if I use Firefox Private Browsing, there isn't any kind of problem. I can open links and then go and open Yahoo mail when using Firefox Private Browsing. I even used Edge and went to AT&T Currently and again I don't have any kind of problem. One thing I did have a problem with was no photos when I would open any link but on both Private Browsing and on Edge using Currently, there isn't a problem. I have spent hours looking on line for a solution but to no avail. Disabling all extensions, plug-ins, diabling my antivirus program and firewall. I believe now, it is a Firefox problem and have even reinstall it twice and same problem. Using Firefox Private Browsing or Microsoft Edge and go to AT&T Currently is a work around solution but it works. I don't like having to do this, but it does work.