Firefox and Chrome are two very popular open source Web browsers, implemented in C/C++. This paper analyzes what topics were discussed in Firefox and Chrome bug reports over time. To this aim, we indexed the text contained in bug reports submitted each semester of the project history, and identified topics using Latent Dirichlet Allocation (LDA). Then, we investigated to what extent Firefox and Chrome developers/contributors discussed similar topics, either in different periods, or over the same period. Results indicate a non-negligible overlap of topics, mainly on issues related to page layouting, user interaction, and multimedia contents.