I would now describe the operating system running on this unit as Raspberry Pi OS (Stretch - Legacy).
To answer the question of mitigation against this freezing situation, firstly, I discovered the 'Resource monitors' applet which can be applied to the taskbar's system tray (rt-click Start Menu Button -> Add/Remove Panel Items -> Panel Applets tab -> Add -> Resource Monitors). This gives a real-time memory usage indication, which is a help.
What really fixes up this situation however, is to use a memory manager. Through 'Pi Apps', I found an installation called 'More RAM' (under the 'Tools' category), which is a Pi-optimised ZRAM install that works by compressing memory. Since then, I've not experienced such freezing at all, even with twice as many browser tabs active.
Pi-Apps: https://github.com/Botspot/pi-apps
Forum post related to 'More RAM': https://forums.raspberrypi.com/viewtopic.php?t=327238
This installation has improved the user experience on my 1Gb Pi 3 no end, so tell your friends; spread the word!