Slow Firefox start-up can be explained by at least the following factors that I know of:
- Growing size of the internal Firefox Database
- Add-ons that are slowing down Firefox
Solution 1 - Optimise databases
To optimise the databases of Firefox, you can use the following script: Firefox optimisation for Linux It simply run optimisation command on the databases that it founds in your user directory. You need to install sqlite3 though, it's available by default in the UBuntu's repositoies.b
Solution2 - an add-on might be slowing down Firefox
For the add-ons, try disabling them one by one and restart Firefox each time. If one is making Firefox much slower then you have to assess: "do I really need this add-on?"