Some reason almoust all modern browsers, but not Seamonkey, are bloatware whose consume all CPU time. Seamonkey is very workable for an old computer.

Only problem with Seamonkey is that it has limited number of language pack, but there is localised versions available too. An example version 2.53.15

https://www.seamonkey-project.org/releases/#2.53.15

Seamonkey can be installed via package management too.

sudo apt install seamonkey

An example Seamonkey´s lightness, almost 20 year laptop with Seamonkey open.

$ inxi -b
System:
Host: antix1 Kernel: 4.9.0-326-antix.1-486-smp arch: i686 bits: 32
Desktop: IceWM v: 3.3.1 Distro: antiX-22_386-full Grup Yorum 18 October 2022
Machine:
Type: Laptop System: FUJITSU SIEMENS product: LIFEBOOK C1110 v: N/A
serial:
Mobo: FUJITSU model: FJNB16F serial:
BIOS: Phoenix/FUJITSU v: Version 1.25 date: 01/14/2004
Battery:
ID-1: CMB1 charge: 34.6 Wh (63.3%) condition: 54.7/54.7 Wh (100.0%)
CPU:
Info: single core Intel Pentium M [N/A] speed (MHz): 600 min/max: 600/1400
Graphics:
Device-1: Intel 82852/855GM Integrated Graphics driver: i915 v: kernel
Display: x11 server: X.Org v: 1.20.11 driver: X: loaded: intel
unloaded: fbdev,modesetting,vesa dri: i915 gpu: i915
resolution: 1024x768~60Hz
API: OpenGL v: 1.3 Mesa 20.3.5 renderer: Mesa DRI Intel 852GM/855GM
x86/MMX/SSE2
Network:
Device-1: Realtek RTL-8100/8101L/8139 PCI Fast Ethernet Adapter
driver: 8139too
Device-2: Intel PRO/Wireless LAN 2100 3B Mini PCI Adapter driver: ipw2100
Drives:
Local Storage: total: 93.16 GiB used: 16.61 GiB (17.8%)
Info:
Processes: 137 Uptime: 40m Memory: 991.6 MiB used: 283.7 MiB (28.6%)
Shell: Bash inxi: 3.3.25