It's unlikely Joss Whedon's Firefly will ever return for season 2, nor that the adventures of Captain Malcolm "Mal" Reynolds (Nathan Fillion) and his space pirate crew will continue in a Serenity sequel.
Whedon's Firefly debuted in 2002 and ran for 14 episodes, ending its run in 2003. Partially modeled on the crew of The Betty from Alien: Resurrection (which Whedon scripted), the series followed Mal and his team on their travels across space in their transport ship Serenity. Incorporating elements from Westerns and various space operas, the series quickly earned a devoted fanbase which continued to grow long after the show left the air. Strong DVD sales coupled with earnest pleas from viewers encouraged Whedon and the Firefly team to make the 2005 feature film Serenity, which deepened the lore and resolved some of the dangling plot threads.
Since Serenity's theatrical release, fans have been vocal about wanting to see even more. This has led to questions on whether a Firefly season 2 or a sequel to Serenity will be made. More than a decade after the release of Serenity, however, it looks like neither will end up happening.
Whedon's interest in returning to his space opera property has wavered over the years. In 2011, he was vocal in stopping a fan-led campaign to have Fillion resurrect Firefly by purchasing the rights to the show. His tune seemingly changed in 2013 when he said he "would love to" get the crew back together. However, Whedon was in between Avengers movies (2012's The Avengers and 2015's Avengers: Age of Ultron), so it's unlikely he would have had time to begin development on Firefly season 2 or a Serenity sequel. Firefly producer Tim Minear was also open to making more episodes of Firefly, even as a limited series. But this was in 2014 and one of the last notable times someone connected to the franchise spoke optimistically about its future.
Fillion also made it clear he wasn't interested in returning for more Firefly episodes in 2016. Speaking at a Firefly panel at Long Beach Comic-Con, he explained, "It's really hard to look at that kind of stuff and say ‘give me more.’ Because enough is enough," adding, "How can everything not be enough?" Fillion's remarks happened during his the tail-end of his tenure on the hit TV series Castle, which was likely a factor in his disinterest to return to the Whedonverse. Even FOX, the network behind Firefly, probably wouldn't have greenlit more episodes unless Whedon was on board.
These days, Whedon and Fillion - arguably the two most important people in continuing the Firefly story on TV or in a movie - are tied up with other projects. Whedon is currently working on the upcoming HBO series The Nevers and Fillion has a starring role on ABC's The Rookie, which is preparing for its second season. The Firefly saga will live on in novels and comics but fans shouldn't expect more episodes of the TV show or a Serenity sequel to happen.