generating php files
The Yahoo! mail app (Mac, Firefox) is generating dozens of files named as.php. Currently, they're going to my downloads folder. Seem to be generated by the ads in the right column. The files register as zero bytes in size, but are a nuisance nonetheless. I sent them all to the trash ten minutes ago, and there already are 42 more.
What's up with that?

It seems like the issue you are experiencing might be a browser-related issue. Indeed those files are generated by ads, but usually, they are not supposed to be downloaded in your downloads folder.
You could try some quick browser troubleshooting steps:
-clear cache and cookies https://help.yahoo.com/kb/sln27197.html and https://help.yahoo.com/kb/sln27196.html
-reset firefox https://help.yahoo.com/kb/sln24149.html
If the issue still persists after that, please try using a different browser(e.g. Safari)