firefox running unbelievably slow on macbook

i borrowed a family member's macbook air after my own laptop was damaged and ive been using it for school for about a year now. it was already old but functional, and i've always ran firefox on it. i was grateful for how easy it was to multitask and have countless tabs open on firefox while working with one or two other programs. nowadays i have only firefox open, maybe two windows, or one window with only a handful of tabs. the decrease in performance is dramatic. even writing this post, theres a delay in my typing and the words appearing onscreen. i noticed that when i have firefox open like this, other programs seem to stall too. i definitely have less content open than before, but firefox itself is sooooo much slower now. what gives? has anyone experienced something similar?