The Mozilla designs are based on single-process operation for the browser. This was an older approach used at one time by all browsers (and all software, since multiple processors didn't exist in those days). Multi-tasking the computer's single processor was used by the software and host computer to run multiple programs at the same time by quickly switching on and off the various individual running programs' access to the CPU core. Unfortunately, with a lot of things running on the computer, a particular program could only receive a limited share of the processor's attention which slowed down the program's operation. Later, by incorporating multiple processors in the hardware, a given program could be given an increased share of a CPU processor's attention and run faster.
Then, with multiple CPU cores available, software began to be written to take full advantage of the added capabilities inherent with the added CPU cores. At some point, many resource-intensive programs began to heavily rely on the presence of multi-processor architecture to maintain their speedy performance. While many such programs can still operate on single-processor systems, they tend to run slower or require the use of less resource-expensive features by the user in order to maintain a reasonable speed. Beyond that, having each tab and extension running in its own process provides a browser with resistance to total crashing if one tab or extension crashes by itself. Unfortunately, the use of these extra processes requires much more sharing of attention when/if a single-processor CPU is used, hence it tends to slow down such a system's overall operation. Mozilla apparently has elected up until now to maintain their single-process design (which tends to be more optimized for single CPU systems), but there are reports Mozilla will also eventually be moving to a multi-process browser design in the not-so-distant future.
There is a sometimes-unpleasant reality that as hardware inevitably evolves in capability, software also evolves to where it relies on the advanced hardware. This leaves older systems lacking in the necessary capacity to run newer software as effectively. Unfortunately, it's simply the way things are in the real world.