It could definitely be a factor. Having more tabs open will generally use more system resources. The help article [[Firefox uses too much memory or CPU resources - How to fix]] contains some suggestions that may help you if you're often finding that your computer is running out of resources. There's not really an 'optimal number' of tabs that anyone can tell you to have open, because it'll depend on how powerful your computer is, what other programs you have running at the same time as Firefox, and what websites you have open (some websites are very light on resource usage and others are quite intensive). I would also recommend that, the next time you feel that the performance seems to 'slow down', you take a look at [[Task Manager - see whether tabs or extensions are slowing down Firefox|Firefox's Task Manager]] and see if there are particular tabs you have open that seem to be consuming an excessive amount of resources compared to the others.