You can't defer upgrades forever. At some point of time Firefox patches from the main branch will also be merged into the long term servicing branch. You'll have to find a fork if you want to continue using XUL add-ons.
Among the forks there's Pale Moon that still has the 32-bit version and the long term plan is to support XUL add-ons forever
Pale Moon supports and will continue to support the following features/technologies:
- Full UI customization
- Full theming (complete themes) and lightweight theming (personas)
- XUL and XBL to build interfaces and applications (including the ability to launch independent XUL-apps from the browser binaries).
- Full support for NPAPI plug-ins
- Overlay and bootstrapped (restartless) extensions
- Access to low-level APIs from extensions, allowing them to truly extend browser functionality, and not just manipulate web content
- Pale Moon Sync (in the secure, time-tested Weave fashion); it will be able to use any Weave Sync 1.1 compatible server, including some FOSS cloud solutions.
Despite that it has been diverged too long ago, therefore the code is now very much different from the current Firefox. And of course it can't run on multiple CPUs.
The developers of Pale Moon are also developing a new browser named Basilisk based on newer Firefox ESR
A XUL-based web-browser demonstrating the Unified XUL Platform (UXP).
This browser is a close twin to pre-Servo Firefox in how it operates.
However currently it's still in beta stage and not considered stable.
That said, the recommendation is to install a 64-bit OS and use a 64-bit browser. Apart from performance improvements, 64-bit apps have bigger address space, which helps ASLR work more efficiently. On 32-bit OSes once an app uses up to near its maximum 2/3GB of memory, there's no free space for it to move around anymore. That's why Mozilla said that
On the flip side, as well as avoiding address space exhaustion problems, a security feature known as ASLR works much better in 64-bit applications than in 32-bit applications, so 64-bit Firefox will be slightly more secure.
Besides, the latest Pale Moon release note also mentioned
This is a bugfix point release to address serious performance bottlenecks and general run-time issues (UI slowness, crashes, hangs) with the browser. Once again this impacted 32-bit operating systems more severely than 64-bit ones due to its more limited address space that would get flooded with bogus data.
Similar thing is said from Chromium's team:
Finally, on 64-bit, our defense in depth security mitigations such as Partition Alloc are able to far more effectively defend against vulnerabilities that rely on controlling the memory layout of objects.
Also read You Should Upgrade to 64-bit Chrome. It’s More Secure, Stable, and Speedy
For various other things that make a 64-bit OS generally safer, read is 32-bit safer