You first need to decide what you are testing, whether its just the PHP render time - or the real-world page load time.
In either scenario, using firebug isn't going to be reliable - as your internet connection itself could be the bottleneck or reason for jitter.
PHP Render Time
If you are purely looking to see if PHP render time has improved/changed - then the most accurate way is to use the output from the Magento profiler.
Admin > System > Configuration > Developer
Make sure that profiler is enabled.
You'll end up with a tabular output at the bottom of every page (front-end and back-end) breaking down the page load time, measured from the point of
Mage::run() starting. The very first line will indicate the total PHP render time (within Mage).
This is going to be your most accurate figure in terms of establishing if your PHP changes are making a difference to page load time, not to mention, it will identify any performance bottlenecks.
PHP Web Server Render Time
The next type of test is to factor in the overheads of the web server itself (but not last-mile connectivity). So for this test to be accurate and not influenced by the 'internet' itself - you should run it on the web server itself.
We use our own utility
mage-perftest (more information can be found here) - which can test pure PHP render time, real-world page load time and even concurrency testing.
To test just the PHP web server render time, you would use (replace the URL accordingly)
./mage-perftest -u me-s1.sonassihosting.com -b
This test will give a breakdown of page load time (for just the PHP element of the page, ignoring any JS/CSS/Images). The output looks something like this,
Total files: 1
Total downloaded: 4K
Avg. page weight: 4.00K
Total time: 0.035s
Min response: 0.035s
Max response: 0.035s
Avg. page response: 0.03s
Test URL: me-s1.sonassihosting.com
Success rate: 1/1 (100.00%)
Real World Web Server Render Time
The final type of test is the time it takes to download the entire page (PHP + static content). Again, you can use
mage-perftest to do this, eg.
./mage-perftest -u me-s1.sonassihosting.com
Avoid any online testing service like the plague
There are some on-line speed test tools like GTMetrix, Pingdom etc. These are not going to give you any kind of accurate result for granular profiling.
They have their place in terms of testing external network connectivity, but are completely useless as a means of checking actual PHP performance. Stick to on-server/local testing for this.
We wrote an article on remote testing and why you should avoid it,
Running Magento in a VPS is A Bad Idea. Others may disagree - but its not a suitable environment for a Magento store for a number of reasons - and we've answered A LOT of questions down this vein, here are a few