Double tap of trackpad not working in Firefox 56+ on Linux

I have a peculiar problem. I am running KDE Neon and despite having 'tap to click' enabled in the OS settings I see that double tapping doesn't select text in Firefox alone. I've confirmed several times that it works on the other browser that I have and also in other applications. However in Firefox alone I see that it's not working. However if I double-click the trackpad buttons instead, it works. If I make many attempts by continuously double-tapping very fast then sometimes in between it does work. And the funniest thing is that triple-tap works and selects the entire paragraph. Double-tap should be selecting the word, isn't it? Please let me know how to get it to work. It's very annoying as I use double-taps a lot.