From Pale Moon project you can download a Firefox version which is highly customized for the current processors. So the performance will be very good comparing to the normal Firefox installation. before download read the system requirement below.
Highly optimized for current processors
100% Firefox sourced: As safe as the browser that has seen years of development.
Uses slightly less memory because of disabled redundant and optional code
Significant speed increases for page drawing and script processing
Support for SVG and Canvas
Support for Firefox extensions, themes and personas
Windows 2000/XP/2003/Vista/Seven, 32-bit or 64-bit (64-bit O.S.es are not natively supported, but the browser will run fine on them)
A 7th generation or later processor with SSE2 support like a Pentium IV or Athlon 64 or later (see list of supported processors) Standard Pale Moon will NOT run on Athlon XP processors!
64 MB of RAM (128 MB or more recommended)
At least 35 MB of free (uncompressed) disk space
Read more and download it