Two items here.
First, Yahoo Mail doesn't always play nice with the image attachments it displays in Firefox. Some will display fine, while others will assume you want to download them, and prompt you to save them. I went round-and-round w/Yahoo's "tech support", to no avail. Harumph. Anyway, Google Mail works fine.
Second, Yahoo Mail has always opened a new window when you click on a link in email. This almost always is not what I want. Since using Mozilla previously, and Firefox currently, I'm addicted to tabbed browsing, and would like to have my links open up a new tab, if they have to open anything at all.
The way to do this in Firefox is a bit awkward, but here goes:
1) In the URL bar, type "about:config". This brings up an internal page with loads of options.
2) Find "browser.tabs.showSingleWindowModePrefs" and set it to true.
3) Now if you go to Edit/Preferences, options will show up to force window opens to a new tab.
Now Yahoo Mail does what I want, for those times that I still use it.