OSS alternative to Adobe reader for filling online pdf forms

Solution 1:

No matter what you do, if you have a PDF form, and you want to fill it out, it will be downloaded in any case; otherwise, PDF would have to be interpreted and presented to the user on the server (this is, in fact, the case if the LiveCycle Forms Server is used on server side, and it would serve HTML5 (nowadays) if you access it via a web browser.

So, the answer is no, there is none (but I will stand corrected if there is). If you insist on filling the form within a web browser, you better stay with Adobe Reader and the Firefox plug-in. You have the best possible functionality, and you will not have problems with smart forms.