Due to a bork in the MCF build system, FMLRenderLibrary isn't in net.minecraft.src for any build of MCF from 1233 onwards. The latest development branch of Mekanism will work with MCF 1291, right up until you break a block with a complex model such as a Metallurgic Infuser, whereupon it crashes while trying to render the dropped item:

Caused by: java.lang.NoClassDefFoundError: net/minecraft/src/FMLRenderAccessLibrary
    at mekanism.client.render.item.ItemRenderingHandler.renderItem(ItemRenderingHandler.java:494) ~[ItemRenderingHandler.class:?]
    at net.minecraftforge.client.ForgeHooksClient.renderEntityItem(ForgeHooksClient.java:136) ~[ForgeHooksClient.class:?]
    at net.minecraft.client.renderer.entity.RenderItem.func_76986_a(RenderItem.java:93) ~[bny.class:?]

In the end, this means I can't use Mekanism v8 together with any mods that explicitly require MCF 1291.

I'm sorry, Waila turned out to be a complete red herring. The real culprit here is the borked build system for MCF >= 1233

Thanks for your looking into this, I'll see if I can fix it

Fixed thanks to @unpairedbracket!

Confirmed, this is fixed as of 749c579. Great work!