While I was searching the Firefox Addons page for the Yahoo!Mail notifier extension, I came across this interesting and very convenient extension. One thing that has always bugged my about Yahoo!Mail is once you are in your Inbox (or any other folder) you have to click on the message to view it. With the AJAX Yahoo! Mail Extension, a small ‘plus sign’ is added next to each message in your Inbox (or other folder) which will allow you to preview the message without leaving the folder.
As a bonus, you can also download attachments from your Yahoo!Mail with just one click. Note: I have not tried this yet as I normally do not get/accept attachments on my Yahoo!Mail account. But again this can all be done directly from the folder you are viewing the message.
But wait there’s more! Want to reply to the message your previewing? With the ‘instant reply’ option located at the bottom right of the preview message you can reply to the message again without leaving your Inbox (or other folder).
This extension works with Firefox builds 0.9.0 up to the current 1.5.0.X builds. There were some issues when users went to Firefox 126.96.36.199, but there are directions on the developers home page on how to get around this. Also, this extensions can be forced (I have done so successfully) with Bon Echo (Firefox 2) with the Nightly Testers Tools extension. Also a nice alternative to the Yahoo Mail Beta.