The answer to whether Linux has software for filling out tax returns until lately was generally a definite no no. However there is speculation that this is about to change as most packages for tax preparation have found to be compatible with Windows and the Mac systems.
TurboTax Deluxe, H&R Block's Tango, and 2nd Story Software's TaxAct -- are all browser-based applications for the preparation of tax that are available as online applications. Though these are best suited for Windows and Mac users these will work just well enough by users using Ubuntu 7.10 and Firefox 220.127.116.11. However two of the three vendors would warn you that these are not supported. In other words the vendor will not help you if you happen to have any problems.
These three tax preparation products would secure a connection to the web sites once you start on your work and it is best to have all the information and the forms such as W-2s, 1099s and financial statements on hand prior to starting out on the project. It will also allow you to log off and reconnect and carry on without a problem.
The three will come up with an easy to follow question and answer format that you answer from the specific questions it will ask you.
Make sure if you intend to use any of these products to file your taxes that the selected product is able to handle all of the forms required to file your taxes. But, firstly try using the product in the free mode before you go investing in the software and find that it is not for you.