Zathura not opening hyperlinks

I am using neovim with zathura as my LaTeX editor. I am using the hyperref and URL packages to insert a link to a website. I know zathura has recognised the hyperlink because it places a box around it (see below). However, clicking on this link does not open the corresponding page unless a firefox window is already open, in which case it opens a new tab with the corresponding URL. Can anyone explain this unusual behaviour?

Note: I am running the Pop!_OS 20.04 (ubuntu based) distro.